RRS "Race signals" seems quite clear to me. Just a question of common sense and courtesy towards the competitors. But you're right: there's no obligation in the rules..

Two comments: 

1. Why would you not display L?  Lost it? Can’t find another one? Don’t have an available halliard? Too busy? If so, with what?   There’s no good reason, probably.

2. The 2018 Q&A is still the best resource on this subject, never mind it has kind of “expired”.

Agreed. Why would t you want to display L to let the competitors know you have posted something, unless you want to create a GOTCHA moment? After all, it's their regatta, and you're there to run it for them.

It's clear from RRS 90.2(c) that the display of flag L is not required.  However, as others have stated it's a very good practice.  I normally have a Sailing Instruction that states that flag L will be displayed over a numeral pennant with the pennant indicating the number of the notice.  With this, the competitors know that another notice has been posted since the last one they read.

Thank you all for your replies.
I'm a competitor myself, and I do believe that the use of the flag L is a very good practice.
I sail Lasers, and as you know, there is non VHF which may help the RC and the racers. That flag is the only way to let all competitors know that the RC has published a new notice.
However, it is still a matter of curtesy, and all your replies and comments seem to go that way.
And, you will agree with me, there is a substantial difference between "curtesy" and "obligation"... ;-)

I agree: there is no practical reason not to fly the flag. But, to understand my question, you should know that it stemmed from a theoretical discussion between a Tutor and his disciple. Perhaps a sterile discussion - it's not the first, and won't be the last on racing rules interpretations, I bet -  but this time it seems that the disciple was right, while the Tutor was not, which is great !!!  :-) :-) :-)  

While ther may be no obligation to display the L Flag, I suspect that most Juries would interpret the failure to display the L Flag to constitute an "omission" if not an "improper action" by the Race Committee under 62.1(a) and might allow a request for redress by a competitor whose place is made worse by the change which she may be unaware of. If the flag is not displayed is it reasonable to expect the boat to check for changes to what has been published?

Thank you Nigel.
That is exactly the point !
What I saw in my competitor's experience is that, in the morning, before the race starts, when the RC is still fresh and crisp, they never miss to display the flag L. Sometimes, as Peter suggests, even flying L over a numeral pennant with the pennant indicating the number of the notice (I'd expect that only in quite important races, however...)
On opposite hand... after the race, when the RC is beaten by a day out at sea, and - most importantly - if/when the RC and the PC are made by the same officers, you may lack the focus and the manpower to hoist the flag.
I saw that happening ... but - as far as I know - no one complained.
The answer to Q&A 2018-005, which I mentioned before, states very clearly that "it is the responsibility of a competitor to check the official notice board", but all your replies imply a question "Why the RC should miss displaying that flag?"
My take away is: displaying the flag L is definitely not an obligation by the RC (so, no redress for lazy competitors who do not read notices, I'm afraid), nevertheless there must be very good reasons not to fly it (tiredness and lack on manpower not being exactly an excellent excuse...). That goes far beyond a mere matter of courtesy: it is crucial for the fairness of the competition.   

It seems to me that the important thing is to be sure that everyone racing gets the message.  There are times when the flag is not needed because you know that everyone will see the notice board or will be told by the race officer, or whatever.  If you're relying on VH\F be sure everyone acknowledges!  There are just as many, or probably more, times when the only safe way is to put up the flag.  Always use whichever is best, but err on the safe side and if in doubt put up the flag and, as I say, make sure that everyone has got the message.

Never rely on VHF: no one knows who's speaking or answering. It can be anybody, including a bored fisherman with no catch...

Thanks for suggesting the numeral pennant for successive Notice postings.
Another easily understood version might be to tie a streamer to the halyard below L for each additional Notice.
(A ribbon, sail tie, surveyor's tape. any tape, or a bit of small rope.)

The L flag has different meanings depending on whether it's flown ashore or on the RC boat. I would think that if notices were posted and L signaled ashore that should cover everything posted before competitors get underway. Seems like L should only be required on the RC boat underway if there's additional information to impart that requires the competitors to come within hail or follow the committee boat. And I agree there's no reason not to fly it even if there's no information to impart.

Related question: If the RC boat hoists the L flag and one or more competitors fail to come within hail, have they broken a rule by disobeying a race signal?