Via [url=http://blog.counter-strike.net/index.php/2018/05/20414/]the CS:GO blog[/url]:

[quote=Valve]The following changes are available in the 1.36.3.5-rc1 CS:GO Beta depot.

[GAMEPLAY]
– General adjustments to ambient sounds:
— Increased resolution and character of reverb effects to better reflect surrounding map geometry
— Small mix adjustments
– Improved vent destruction sounds and increased audible distance.
– Added a new hitbox type for necks and applied it to the existing neck hitbox on character models. Bullets that penetrate the neck into the head will now count as headshots. This should make it more intuitive to tell when an attack from behind will result in a headshot.
– Fixed ‘use’ key to more reliably pick up weapons on the ground and to not drop the active weapon if a replacement weapon cannot be picked up.

[MAPS]
Nuke:
– Optimizations for low settings
– Blocked visibility between silos inside A site
– Ported over ShortNuke gameplay changes:
— Removed Toxic
— Vent no longer connects directly to B site, but has another exit close to double doors
— Pushed new corner boxes on B site further back to prevent headshot angle towards ramp
— Added solid background to yellow crane inside B site
— Smoothed out some player movement

Overpass:
– Lowered volume of ambient sounds in Overpass
– Added grenade and player collision to fence cage near construction
– Removed spooky shadow fading in when approaching T main entrance to B site[/quote]

Rumor has it:

- Wish to opt in? Right-click on CS:GO in your Steam library, go to Properties, go to the Betas tab, then select "1.36.3.5-rc1)"
- To opt in for servers: run "app_update 740 -beta 1.36.3.5-rc1" in SteamCMD
- Valve servers will be unavailable as has been the case with the last few prerelease tests

- SteamDB GameTracking services will be completely unavailable as it only keeps eyes on the public branch - be aware

- Size is ~85 MB