I have raised the question about such a policy with the governing board. While there isn't a decision, the Foundation side was able to share some information about the current "rule of thumb" policy in place, including some considerations taken into account.

Upon further discussion in the "Office of the Matrix.org Foundation" room, RichvdH shared this page from the Element backend team's (who are contracted to operate the matrix.org homeserver) internal handbook: https://gist.github.com/richvdh/d7e83516207efbe9b31c4e2529a4132d

The discussion has led to the provisional agreement that we add the suitable parts to an appropriate place under matrix.org/homeserver, after which the backend team will reference it from their internal docs and also maintain it (we should hence add them as codeowners of that section as we add it).

The backend team presumabily is @matrix-org/synapse-core.