IRC has a rating rule regarding the specific gravity of the material used in the keel, centreboard, and rudder.  The below is from 2023.  The original change was related to a maxi named Alfa circa 2005 that put tungsten ingots in the keelbulb if I remember correctly?  Note that tungsten is not a workable metal except by mills, the equipment required is very specialized.  You may want to declare what you have doing so it is above board.  Unless you make a keel in gold, then just show it off with the occasional short haul.

https://ircrating.org/irc-rule/

19 HULL APPENDAGES
19.1 Full details of a boat's hull appendages shall be supplied to the Rating Authority at the time of rating application.
19.2 Any keel fin fairings with a nominal density, including any cavities and/or core material, significantly lower than the main structural elements of the keel fin shall be declared. For the purpose of this rule, a keel fin fairing does not include surface fairing, filling and painting materials up to a total thickness of 10 mm, measured normal to the local surface of the keel fin.
19.3 Drop keels, centreboards, bilgeboards and other moveable hull appendages shall be declared. Unless fixed down while racing, drop keels will be rated as moveable hull appendages.
19.4 In the construction of hull appendages, no material with specific gravity greater than 11.3 is permitted.
19.5 Boats including material in their hull appendages with specific gravity greater than 11.3,  and with age date of 2005 or earlier, and holding a valid IRC certificate on 31st December 2005 are exempt from Rule 19.4. Apart from maintenance, any such boat changing the quantity of material of specific gravity greater than 11.3 shall comply with Rule 19.4 and will not subsequently be eligible for exemption from compliance with Rule 19.4.
19.6 Any material in the keel fin of a keel type 10, 11 or 12 with a specific gravity greater than 8.0 (eg. lead) shall be declared in writing 

I can fi d no mention in RRS or ERS, but some web searching found quite a few sets of class rules with it in, so I conclude it's a class rule issue. 

The issue first appeared in the 80s or 90s when one of Taberly's 'Pen Duik's' appeared with depleted uranium ballast, courtesy of the French Navy.  The rating rule of the day (IOR?) quickly outlawed it, with a grandfathering or sunset clause for Pen Duik as I recall.