A curious entry appeared on Steam for Half-Life: Alyx: a branch named previous_release now holds the current stable build. Players noticed it quickly; branches like this often show up when something bigger is being prepared.
Valve hasn't commented. The most straightforward guess is a technical update in the works — think engine changes or added support for a new Steam VR headset (e.g., driver or API updates). Keeping a previous_release is also a practical backup for mod users if an update breaks compatibility.
That practical take hasn't stopped wilder readings. People remember similar changes around Portal shortly before the announcement of Portal 2, which later became part of the campaign, so some hope this is more than maintenance and might point to future Half-Life plans — or it's just wishful reading.
None of these theories are confirmed at the moment.