Via the Steam Community:
Valve- Added 64-bit support for Windows/Linux client and server
- Should include performance improvements for most users
- Bugs can be reported here: https://github.com/ValveSoftware/Source-1-Games/issues
- Fixed an exploit related to uploading invalid custom decals that would crash other clients
Rumor has it:
- avast!, AVG, and Bitdefender users should all be aware that false positives are likely to be triggered with the x64 .exes included as part of today's update (including vrad.exe and shadercompile.exe)
- Instructions on reporting affected files as false positives can be found here
- As mentioned above, the addition of 64-bit support is exclusive to Windows and Linux, and the x86-64/AMD64 architecture
- The status quo of TF2 being unplayable for macOS users on Intel Macs running macOS Catalina (10.15) or later, and Macs with Apple Silicon still remains true after this update. There is no word on whether an AMD64 and/or ARM64 port will occur for Mac users, but given inaction up to now plus Counter-Strike 2's abandonment of the Mac, it appears unlikely
- Additionally, a round of updated localization files have snuck in, made possible in part by contributions from Translators Like You - Thank You
- Size is ~315 MB - data capped users should be aware