The option to revert to an older build of a game has been an undocumented feature of Steam for a while now, but as SteamDB notes, the latest beta changes how the client connects to its content delivery network, removing the access customers had to previous versions of their games.
Going back to an earlier build wasn't something every Steam user was likely to need—in 2020 what was previously a command line function was quietly changed to require a third-party tool—but it still had value. Updates to games like Grand Theft Auto 4, Beat Saber, Dragonball Z: Kakarot, and various Bethesda RPGs have broken mods and required either updates from the modders or rolling back to an older build. Some speedrunners and achievement hunters rely on access to specific versions of games, and they're also worth having from an archival perspective. When an update to Age of Empires 2: Definitive Edition made old replays unwatchable, casters had to downgrade to a previous patch to get them back.
It's worth pointing out that this is different to when developers give access to variant builds via beta branches, which won't be affected. Even if this change does go ahead in Steam's stable version, you should still be able to access the builds you need to play Euro Truck Simulator 2 and American Truck Simulator in VR and enable split-screen co-op in Resident Evil 5, for instance.
“We do not know if Steam CDN will enforce this requirement for all games (if at all), or what other requirements may be”, the SteamDB team writes, while also noting, “This change will impact how SteamDB operates, and we will no longer be able to fully track file lists of all games on Steam”.
According to the beta patch notes, the change will “Ensure that all installed and Cloud-enabled games are synchronized as soon as possible, to avoid delays when launching games and to prepare for possible offline play.”
We've reached out to Valve for comment and will let you know if we receive a reply.