2015: Experimental by default
While the new world was unfinished, the old exe still launched. Patches, anti-cheat, the server browser — all went to Experimental. Legacy froze: 2015 bugs stayed 2015 bugs.
December 2016: gone from Steam
Facepunch removed Legacy from the store and library. If you had not kept the folder, the client was gone. Official dedicated for the old protocol is dead. Playing Legacy since then means a third-party exe plus someone else’s (or your) server.
Legal meaning
Facepunch owns the game. A fan server and launcher sit in a grey area: you cannot ship their client as “our product” or pretend it is official. This site is a project page and a reference, not a Facepunch storefront or a Steam mirror.