MSC: Removal of query string auth

proposals/4127-remove-query-string-auth.md

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+# MSC4127: Removal of query string auth
+
+See [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126) for details on query
+string authentication and the prerequisite deprecation.
+
+## Proposal
+
+Query string authentication becomes *removed* from the [Client-Server API](https://spec.matrix.org/v1.10/client-server-api/#using-access-tokens)
+and [Identity Service API](https://spec.matrix.org/v1.10/identity-service-api/#authentication).
+
+The prior deprecation required by the [deprecation policy](https://spec.matrix.org/v1.10/#deprecation-policy)
+is described by [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126).
+
+As a process note, it was considered to bundle the deprecation and removal into MSC4126 instead of
+having two separate MSCs, breaking the deprecation policy slightly, though this introduces consequences
+which are, in the author's opinion, undesirable. Specifically, the spec release changelog for the
+deprecation would say "as per MSC4126" and imply the MSC being moved to the [merged](https://spec.matrix.org/proposals/#process)
+state, however the second half of the MSC (the feature removal) would not have landed yet. This would
+leave the MSC stuck in an awkward `spec-pr-missing` state because half of it had not been written up
+or landed yet. To maintain a clearer changelog, ease process considerations, and give clients another
+opportunity to object to the removal, the two MSC approach required by the deprecation policy is best.
+
+## Potential issues
+
+See [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126).
+
+## Alternatives
+
+See [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126).
+
+More of a process note, during drafting of this proposal the author considered combining the deprecation
+and removal into a single MSC, against the advice of the deprecation policy. The aim was to reduce
+process burden and more quickly get the security risk removed from the specification, though merging
+both operations into a single MSC potentially makes the spec release changelog unclear and leaves the
+MSC in an awkward released-but-not-merged state. To avoid figuring out how to navigate the process
+states, increasing the process burden rather than decreasing it, the deprecation policy's two MSC
+requirement has been followed here. However, there are still other process efficiencies which can
+take place - see the "Dependencies" section of this MSC for details.
+
+## Security considerations
+
+See [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126).
+
+## Unstable prefix
+
+This proposal cannot feasibly have an unstable prefix. Clients are already discouraged from using
+query string authentication and should switch to `Authorization` as soon as possible, regardless of
+this MSC.
+
+See [MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126) for related details.
+
+## Dependencies
+
+[MSC4126](https://github.com/matrix-org/matrix-spec-proposals/pull/4126) must be *released* in the
+specification before this proposal can be merged, though this MSC *may* progress through Final Comment
+Period concurrent to MSC4126 if desirable.