With the exception of spotlight ladders, old gen formats are typically challenge only, due to the fact PS focuses mainly on current gens. This includes Gen 8 as the most recent removal, with some exceptions as decided by format leaders. Not all formats playable on PS! can have a permanent ladder; recency of gen, popularity, and official status all go into consideration of whether or not to implement one.
If you don't see a ladder for a format you'd like to play, you can challenge someone to a battle directly or play a room tournament. You can also ask for old gen battles in the Ruins of Alph room and OM battles in the Other Metagames room.
Gen 8 Doubles Ubers is one of the Gen 8 formats whose ladder got removed due to Gen 9 being the current gen. If you want to play Gen 8 Doubles Ubers, either challenge someone to battle in that format since it is currently a challenge only format (unless its a RoA spotlight format), or ask for a Gen 8 Doubles Tour in the rooms mentioned in the quote.
Source: typing /rfaq in the Help room on Showdown
Hope this helps!
