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.
too bad there are no server stats displayed - did anyone play it yet, are there people online, at certain times at least?
Last time I tried there were. Not a big fan of this type of game though.