Comment by Arathorn

This will be referring to enforcing MSC4311 (https://github.com/matrix-org/matrix-spec-proposals/pull/431...) stripped state validation. Synapse announced a 1 year compatibility window to avoid ecosystem fragemtnation: https://github.com/element-hq/synapse/issues/19943. However, due to MSC4311 not better outlining how to handle compatibility, another server implementation chose to enforce the MSC more rapidly, breaking compatibility with everyone who hadn't yet implemented the MSC - including Synapse, which hadn't actually implemented it yet. Speaking as the lead of the Matrix Spec Core Team, we should have handled this better. https://github.com/element-hq/synapse/pull/19723 is the Synapse implementation which is now in the process of landing.