Consider a subspec of some kind for standardizing X over Matrix, where X isn't really core

Nobody has claimed this yet.

*I'm certain we have an issue for this, but I can't find it.*

We have several ideas of things which might be standardized using Matrix so other Matrix consumers can integrate safely and efficiently, however adopting them into the core specification is awkward at best.

So, what if we just made them a subspec of some variety? We'd link to them from spec.matrix.org somehow (maybe even manage them under the matrix-spec repo itself), and consider them something people can submit MSCs to, etc.

Open questions we've thought about loosely but don't have answers to are:
* Who owns these "vendored specs"? Is the SCT really in the best position to talk about 3D worlds, or should that be someone else? Do we need to formalize committees within the Foundation?
* How are changes made to these specs? MSCs?
* What do they look like? Where do we draw the line? What don't we want to accept, if anything?

**We are not expecting answers to these at this point, and honestly wouldn't expect feedback until a draft MSC was put up.**

Current examples:
* State counters used by element-web
* https://github.com/matrix-org/matrix-spec/issues/973
* https://github.com/matrix-org/matrix-spec-proposals/pull/3815
* https://github.com/matrix-org/matrix-spec/blob/main/informal/idp-brands.md

Start by reviewing the linked examples: state counters in element-web, issue 973, pull request 3815, and informal/idp-brands.md. Done would be a draft MSC that defines the proposed subspec model, including ownership, change process, scope, and acceptance boundaries.

Written by the indexing model from the issue text.