Afraid not R_P - same applies to the club_id. I remember this same question being asked years ago and the answer was no, there's no way of accessing any related endpoint using player or club id's.
Shame because a fixed reference we could use to do that would be very useful.
Weirdly no. Which is annoying, as when a user changes his/her name you lose the link.
The best I could come up with is to store the ID, the link to the best game and what colour they played. So when the link is broken you can check that game and find out the new user id...
Or you can just have the ID and the link, then check which player has that ID.
I use a similar system but I maintain a list of team matches in which clubs have played and no matter how many times a club changes its name, the new one can always be found by examining the name of the relevant team in its given match.
Of course you have to record a suitable match id on the day you're certain of the club's name.
If you know the username of a player, you can find his/her user_id simply in the player endpoint. Is there some way to do this the other way round: finding the username that corresponds to a given player_id?