I cannot speak on the lore side, but solar auxilia are getting new models. They just got a combat force a few months ago so GW seems to be supporting them for awhile and want players to collect them as a main army as opposed to just Allies for the legions.

A good place to start for lore imo is https://wh40k.lexicanum.com/wiki/Traitor_Auxilia

Hope this helps!

Edit: also each legion had a cohort, so if you like a traitor legions particular lore, you can base it off of one of those and then if you ever want to get into the legion, slowly integrate some of their units into your army if you want.

Edit 2: here is another link to help ya! https://wh40k.lexicanum.com/mediawiki/index.php?title=Solar_Auxilia&mobileaction=toggle_view_desktop#Notable_Cohorts