We’ll need to think harder before implementing something like community.lexicon.service.describe on the Bluesky PDS distribution as it’d probably become something of an implicit standard at that point.
In the mean time & to unblock things, how about just adding a spacesAlpha boolean to the com.atproto.server.describeServer output? (I might not even put it in the lexicon)
One step further could be experimentalFeatures: ["spaces-alpha"]