Filter lobby opponent by flag

[ddugovic]

Players in warring nations may wish to "opt in" to filtering (excluding) opponents by flags when playing in the lobby:
https://lichess.org/forum/lichess-feedback/avoid-russians-in-match-making
https://en.wikipedia.org/wiki/Category:Lists_of_wars_by_country

[ddugovic]

I think the best solution would be to simply remove the ability to specify nationality from the profile section in the system. Why should nationality matter? We just want to play chess here. No more flags in profile.

https://lichess.org/forum/lichess-feedback/how-can-i-block-all-players-from-russia?page=3#29