MSC3589: Room version 9 as the default room version

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. 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 does not currently have a published specification, though the details are recorded at
+https://github.com/matrix-org/matrix-doc/pull/3387
+
+## 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.
+
+Prominent server implementations besides Synapse have partial or no support for v9. Namely, Conduit
+is lacking support and Dendrite appears to have most of an implementation but is not complete. For
+these projects, and other servers in the wild, it is encouraged that implementation progresses to
+support v9 fully.
+
+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.
+
+## 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.