This isn't true.Then be the change you want to see and start one on Discord. It'll start out challenge-only, and if it gets popular it'll become supported on Showdown.
The only reason such a group doesn't exist is that none of the ban-tera advocates care enough to invest the time and effort to make it happen.
Love him or hate him, LosetoRu? did submit a formal application to create a Tera-free OU room on PS a while ago. I don't have the link to the application itself but he mentioned it here. You don't have to agree with the concept, and it got shot down by upper staff, but it's wrong to say that "none of the ban-tera advocates care enough to invest the time and effort to make it happen." I agreed to moderate that room. We did try.
Idt there's any precedent for a challenge-only format becoming popular enough to have a permanent ladder on showdown, it feels disingenuous to suggest this. Correct me if I'm wrong.
Realistically, all that ban-tera advocates can do is get good and wait, please join me one and all.