Closes: #18436 Implements: https://github.com/matrix-org/matrix-spec-proposals/pull/4308 Follows: #18674 Adds an extension to Sliding Sync and a companion endpoint needed for backpaginating missed thread subscription changes, as described in MSC4308 --------- Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org> Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>