I'm not really disagreeing with your point but I think in general Valve has kept their games that share an engine on the same trunk, so TF2 should be on a pretty recent version of source. The update accompanying this release includes the bicubic lightmap and radial fog implementations from the HL2: 20th Anniversary update.

For Source 2 it ends up being a constant source of leaks where strings for engine features of in-development games get shipped out in Dota 2 and CS2 updates. We learned tons of stuff about Deadlock that way.

https://www.teamfortress.com/post.php?id=238643

Yeah last year Team Fortress 2 got the 64-bit update and replaced some commercial libraries. The branch of Source that it's on seems to get periodic security updates too.

Are you considering Source 2 in this assertion? That piece of software has received quite significant development effort.

I'm not. Source 2 is very modern even by caveman gamer standards.

Source got some significant updates such as HRTF audio in 2016 and Panorama UI in 2018. I don't know if TF2 specifically ever got those changes, though, but Source engine itself did as used by CSGO.