Thanks Giles, that puts the case very well. An endpoint accessed by club_id would be immensely useful & overcome all the problems we experience when admins change the names of their clubs without warning. As we've been told club names being mutable are identifiers that we shouldn't rely on anyway.
So that points to the need for an alternative, in other words the club_id.
This follows from discussion in notes, transferring here @bcurtis' request.
1. Having player_id and group_id be guaranteed to be continued to be provided(*) would be of benefit as account names may both be changed and reused, and prior to club_id appearing (perhaps it was always there, and I missed it?) there was no way to track group name changes.
Club name changes are critically important for the many team-based competitions on chess.com, such as TMCL (Team Match Championship League) that @stephen_33 helps run.
(club_id 35246). (OK, https://api.chess.com/pub/club/team-match-championship-league)
Group name changes are not uncommon: France/Deutschland Group changed to France-Deutschland Group to make something easier (I don't recall the details) and Great Viking Warriors changed name temporarily to Asger's Great Viking Warriors and then decided to make the change permanent. A number of Australian city/state teams regularised their names. (To a format I don't like, but hey, wasn't my decision.)
User name changes are not uncommon either, and it is useful to have an unchanging reference when tracking members (who has played in what matches?) if any cached data is involved (and long-lasting groups play a lot of matches).
A consistent player_id is also useful when tracking potential trolls, abusive members and cheats
2. An endpoint to find a member by player_id saves having to stumble across them, or else try to track every active chess.com member and their current profile
3. An endpoint to find a club by club_id would ease running competitions via tools using api.chess.com, as the club_id can be recorded as the competition entrant and competition administrators don't even need to be told of club name changes; they will be obvious.
From the current documentation:
(*)Note: the "player_id" is provided as a convenience to determine when a username has been changed. If you retrieve a Player Profile by the username-based URL linked from a Game or other object, and this new Player Profile has a "player_id" that matches a Profile you previously downloaded, then you can safely assume that this new Profile replaces the old, and all URLs with the previous username will now be found under the new username. This should be an extremely rare occurrence. This "player_id" will never change for a given account, however the future availability of this ID is not guaranteed.