Why was US/UM Monotype taken down?

Question

In US/UM Mono, you could have virtually every Pokémon on your team as long as they followed the clauses. Gen 8 does not have any National Dex, and I'm limited to only the Pokémon in The Galar dex, Crown tundra dex, and Isle of Armor dex. Why did they make US/UM monotype unavailable to play?

Comments

You'll likely get a better answer if you ask here: https://www.smogon.com/forums/threads/ask-simple-questions-here-read-original-post-before-posting.3520646/ I'm pretty sure this is just because very few people are still playing gen 7 Monotype.

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Normally its cause they move on, so it may be in roa spotlight eventually if you want to play it.

Answer 1

Old gen formats are challenge only, due to the fact PS focuses mainly on current gens. This includes Gen 7 as the most recent removal, except for OU, Ubers, Anything Goes, 1v1, and Doubles OU. You can click on someone's name and click challenge, do /challenge name, format to challenge them to this tier, or play a room tournament. You can also ask in the Ruins of Alph room for old gen battles. These formats will not be coming back unless Ruins of Alph decides to put them on the Monthly Spotlight.

Source: Typing /rfaq oldgens in Help room.
As Sleepy Dreepy already told, they will be playable once RoA adds them, another reason could be that it's not as much popular as other Gen 7 Formats.
Hope this helps.