MSC3589: Room version 9 as the default room version (#3589)

* Room version 9 as a default room version

* Update statistics and notes about maturity

* Adjust wording for Conduit

* Update to stable link

proposals/3589-v9-default-version.md

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+# MSC3589: Room version 9 as a default
+
+Enough time has passed to allow the public federation to upgrade their servers to support room
+version 9, though with some caveats (see "potential issues"). This proposal aims to make v9 the
+default room version.
+
+## Proposal
+
+The specification adopts v9 as the suggested default room version, making no changes to the stability
+of any room versions. As of writing, v6 is currently the suggested room version.
+
+Room version 9 is currently published here: https://spec.matrix.org/v1.2/rooms/v9/
+
+## Potential issues
+
+Servers will be encouraged to update their config/internal defaults to use v9 instead of v6. This
+is considered a good problem to have.
+
+Note that servers are not required to honour the default room version due to it being a suggestion
+in the specification, however they might fall behind as other servers set their defaults accordingly.
+
+Some server implementations, like Synapse, support configurable default room versions: servers which
+have set this flag will not necessarily be affected by this change.
+
+As of writing (Jan 26, 2022), some prominent server implementations are only just getting support for
+the room version. Namely, Dendrite released a version with support about 2 months ago and Conduit is
+working on including Ruma's changes which landed also about 2 months ago.
+
+Though both Dendrite and Conduit (meaning Ruma) have not had the same amount of time as Synapse to
+reach maturity on v9-capable versions, both the Dendrite and Conduit communities appear to update to
+newer versions much more quickly. This implies, without statistics to back it up, that nearly all
+Dendrite servers out there will have upgraded and that a similar percentage of Conduit servers will
+upgrade once available.
+
+No major issues appear to be reported to Synapse or Dendrite with respect to v9, however Conduit
+has [reported](https://gitlab.com/famedly/conduit/-/merge_requests/257#note_814327701) that the auth
+rules for v9 might not be perfect. All of this indicates to the author that the implementation is at
+least sane and accomplishable, even if a bit difficult to incorporate.
+
+For completeness, some links:
+
+**Dendrite**:
+
+* Tracking issue: https://github.com/matrix-org/dendrite/issues/2010
+* Library support: https://github.com/matrix-org/gomatrixserverlib/pull/279
+* Release: https://github.com/matrix-org/dendrite/commit/b4a007ecceafd2b93fee2e775b0a61283d4a3844
+* Testing: https://github.com/matrix-org/complement/pull/220
+
+**Conduit**:
+
+* Tracking issue: https://gitlab.com/famedly/conduit/-/issues/161
+* Library support: https://github.com/ruma/ruma/pull/771
+* Release: N/A
+* Testing: N/A
+
+## Alternatives
+
+None relevant.
+
+## Security considerations
+
+None relevant.
+
+## Unstable prefix
+
+None relevant - servers can already choose a different default room version legally. This MSC
+just formalizes v9 as the default.