Available from https://flathub.org/apps/details/jp.yvt.OpenSpades #OpenSpades is a free and #OpenSource online first-person shooter created by Ben Aksoy, featuring fully destructible terrain and plenty of game modes (including capture the flag) created by the community.
I used to play this game and it is quite fun, but unfortunately the few times I played recently the server I was on was infested with actual nazis (might vary by server but there aren't many servers that people still actively play on.)