Valve have just updated the Source 1 SDK on GitHub to the latest code based on Team Fortress 2, effectively making TF2 code source available

Good news! TF2 isn't going to die, TF2 is going to live forever!

the source game engine is not open sourced, but tf2, which is built ontop of the engine is … what about Art assets?

The majority of items in the game now are thanks to the hard work of the TF2 community. To respect that, we're asking TF2 mod makers continue to respect that connection, and to not make mods that have the purpose of trying to profit off Workshop contributors' efforts.

Official valve announcement: https://www.teamfortress.com/post.php?id=238809