What is the difference between Legacy and Unrestricted and what would the most recent version on steam be considered?

Legacy and Unrestricted only refers to which glitches and skips can be performed in the speedrun. You can read which glitches these are in the category rules.

You can use the Steam version with any category but some glitches have been long patched in the most recent version. That's why the vast majority of speedruns are performed on older versions of the game, specifically either build 3420 for Out of Bounds category or build 5135 for everything else. You can download 5135 by following the link to "Source Unpack 2.4" in the Resources tab on these leaderboards.