Via [url=https://list.valvesoftware.com/cgi-bin/mailman/listinfo/hlds]HLDS[/url]:

[quote=Valve]- Fixed getting the Masked Mann achievement on maps other than Mann Manor
- Fixed a bug where the grappling hook would not switch back to the previous weapon after being released
- Fixed the Quickplay server list sometimes displaying the map filename instead of the proper name
- Added tournament medals for UGC Highlander Season 15, UGC Highlander Season 16, UGC 6v6 Season 17, UGC 6v6 Season 18, UGC 4v4 Season 4, and UGC 4v4 Season 5
- Updated the Monoculus and Merasmus battles on Eyeaduct and Ghost Fort to call a temporary truce between the teams while fighting the boss
- Added server convar tf_halloween_allow_truce_during_boss_event for community maps to use this feature
- Updated Merasmissions to include a Quickplay button that will automatically join the required map or open the Quickplay menu if any map can be used
- Updated taunts to prevent movement during the pre-round period when players are frozen
- Updated the Invasion Coin to track stats through November 6th, 2015
- Updated pl_millstone_event
- Fixed getting out of the map using spells
- Monoculus moves now around the end area of the map instead of staying near final pit
- Made spells spawn a bit less frequently
- Fixed ghost long arms, now it travels/arrives to hell
- Performance optimizations
- Updated the navigation file[/quote]

Rumor has it:

- Here's the item schema update with the (recycled from previous seasons) UGC medals: https://github.com/SteamDatabase/SteamTracking/commit/e40d284744d61acad8bdaa7187c5e26ed62f8c44.diff

- VPK contents changes and HUD file changes: [url]https://github.com/SteamDatabase/GameTracking/commit/4d3a3636b4b9bef4af9526abddbd4f090240823c[/url]

- Size is ~55 MB