In reading rule 90.3 (e), it seems to me that (if this 24-hour time limit has been invoked in the NoR), the Race Committee cannot abandon a race after that time limit has expired. OTOH, they could request redress of the protest committee, simultaneously asking them to extend the protest time limit.
If the time limit is extended and the the Protest Committee decides that conditions for granting redress have been met, then the Protest Committee could abandon the race.

No I do not see it like this.
The protest time limit is set in the SI it can be extended to validate a protest but there is always a base protest time limit.

Michael Butterfield, perhaps I should have been more detailed or explained some other assumptions.
From Matt's question, I assumed that there should have been some reason why the Race Committee only decided at such a late date that a race should be abandoned. Usually, they know that much earlier. I said "protest time limit" because the time limit for a request for redress is "the protest time limit or two hours after the incident, whichever is later." (rule 62.2).
Rule 62.2 also says, "Other requests shall be delivered as soon as reasonably possible after learning of the reasons for making the request. The protest committee shall extend the time if there is good reason to do so."
In my mind, even if new rule 90.3(e) is in effect, if the Race Committee acts promptly to request redress for a large number of boats (even after the protest time limit has expired), the protest committee can act to abandon the race as long as the conditions for redress are met and that is the most equitable arrangement for all the boats affected (rule 64.2).

Again it refers to the protest time limit just because you can have redress "two hours after the incident" does not change the protest time limit and it is this that is referred to in 90.3.

Michael:
Just so I understand, if:
- The Notice of Race has stated that rule 90.3 (e) is in effect;
- The last race of a series is completed and protest time has expired; 
- 24 hours have elapsed; 
- After that time the race committee discovers that it has made an improper action or omission that made the scores for several boats worse (through no fault of their own); and
- The race committee immediately makes a request for redress to the protest committee.
Then it is your opinion that the protest committee cannot order that the race be abandoned.

Yes that is my opinion. They can have a request hearing, but because of 90.3 the only request result, if granted, is to let the results stand. In any event, it cannot change the scores.
To me, the idea is so you can have a prizegiving with no possibility of a change later.
Again I do not think the results can then be changed if a protest in the event goes to appeal and is 
changed.
It is to give certainty to the results, no matter what may happen.

Michael:
I understand your opinion better now, except that the last sentence in rule 90.3 says, "However, in exception, changes to scores shall be made resulting from a decision under rules 6, 69 or 70. The notice of race may change ‘24 hours’ to a different time." Since rule 70 is Appeals and Requests to a National Authority, I think that even if rule 90.3 (e) is listed in the Notice of Race, a successful appeal can change the scores, as well as an infringement of the World Sailing Regulations listed in rule 6.

Yes I was in error appeals RRS 70 can change scores.

I'm not speaking to a request for redress to the PC.  Rule 32 gives the Race Committee unilateral power to abandon the race at any time after the starting signal (for specific reasons).

Does 90.3(e) restrict this unilateral power?

I would argue that the key question is when should the NoR put 90.3(e) into effect.
An understandable wish to get on with the prize-giving is not a legitimate reason - mainly because I know of no club or event that waits 24 hours to hold the ceremony.
There may be a need for using this rule when the result of the event has an implication for another event. In this case, by putting a time limit on changes to the results this gives a clear indication to the selection committee, or to the competitors, that the results are confirmed and that they may act accordingly.
This rule would have been useful in one case - the race committee made an error in scoring the last race of the final series in a youth team racing event. The error gave victory in the event, and qualification for the European Championships to the incorrect team. By the time the error was noticed, an hour or so after the prize-giving, it was too late - the parents of the winning team had already booked flights and hotels... This created a lot of ill-feeling in the class.
Therefore, I think that we should use this rule sparingly. It's main utility will be to create a safety valve for race officials to double check results, and give competitors, their support teams and also class administrations a clear indication as to when results can be acted upon.

To answer Matt's question, I think the answer is yes, this restricts the power of the race committee. I don't think 90.3(e) conflcts with 32.1(d), it just puts a time limitation on it.  This could be useful for ladder or qualification events where it is important to know exactly when the results not longer subject to change.

I do see an issue with how 90.3(e) is written. Is the 24 hour time limit based on the earliest of the 3 events (protest time limit expires, protest results communicated, and results published) or the latest of the 3 events. I assume it should be the latest, but I'm not sure that can be directly determined based on the wording.

I think that it's the earliest of the three conditions because "or" is used to separate them. Therefore, if any one of the three conditions is fulfilled, then the requirement is met.

The Submission supporting the introduction of rule 90.3(e) was Submission 193/19

It's worth a read.

Firstly the two subparagraphs containing the conditions are NOT joined by a conjunction (and/or).  As I understand it as drafted,  this means that both conditions must be met, which, in effect means the later of the two.

The change, as published added a further condition 'the results are published', and preceded this subparagraph by 'or'.

I think the better construction remains the latest in time of the three conditions.

If the NOR just provides that there shall be no changes to race or series scores in accordance with rule 90.3(e), I think this should be construed as the latest of the three conditions.

However, rule 90.3(e) would also now appear to empower the NOR to specify any one alone of the three conditions as the cut-off time.  I think this would be a bad idea, but nevertheless it appears possible.

With respect to the interaction between rule 90.3(e) and rule 32.1(d), the Submission sort of addresses the possibility of actions after the cut-off time, with reference to a protest hearing.  It says

I think a race committee could still exercise it's power to abandon a race under rule 32.1(d), but rule 90.3(d) would prevent this abandonment being applied to the results.

If an OA has applied rule 90.3(e), I think it would be a bad idea for the race committee to abandon a completed race after the cut-off time, because this would appear to the competitors and the public as contradictory, and defeat the purpose of rule 90.3(3).

If, however, such an abandonment came before a protest committee in a redress hearing, I agree with Mike B, that redress should not be given so as to change the results.