The closest thing I know as a kind of FOSS project (totally not FOSS) is Road to Vostok. It’s still in development, programmed in Godot.

Oh wait!! You may like S.T.A.L.K.E.R. GAMMA. I think all the source code has FOSS kind of licences. The game is free as in beer too. Prettt good actually. Single player inmersive tactical postapocaliptic shooter. With good grahpics too.

Non FOSS tactical games that you may like:

• Swat 4, if you mod it with SEF and use voice commands it gets pretty good.

• Arma 3. You have plenty of mods to make it as realistic as you want. Ace, etc.

• Arma Reforger.

• Ready or Not. Combined with voice attack it’s amazing.

For multiplayer games that aren’t that tactical but still more tactical than FPS games. But that are super fun:

• Rising Storm Vietnam. SUPER SUPER FUN GAME. Coordinated team play is not that important.

• Squad: SUPER SUPER FUN. More realistic than RSV. It has vehicles. Very important to have a good coordinated gameplay in order to be enjoyable.

• Insurgency (Saga): Kind of tactical CSGO (simplified explanation). Cool concept cool gameplay. But the comunity has shifted (in the Insurgency Sandstorm) from tactical gameplay to fps/cod/csgo like. Full rushing and stuff. Skins and that kind of stuff that kills a game. I don’t play it anymore.

Upcoming tactical shooters:

• Stalker 2.
• 83’.
• Arma 4.

Others i haven’t played:

DayZ.

Games that are good but that i don’t recommend at all because of its devs:

Escape From Tarkov.