Add ADR for extracting unstable features

adr/2024-05-08-extract-unstable-keywords.md

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+# Extract Unstable Features for Initial Release
+
+* Status: accepted
+* Deciders: @gregsdennis @jdesrosiers @bhutton
+* Date: 2024-05-08
+
+Technical Story:
+
+@gregsdennis [proposed](https://github.com/json-schema-org/json-schema-spec/issues/1443#issuecomment-2099427543) that an ADR be created to document that unstable features be extracted before the stable release.
+
+Also the [SDLC proposal](https://github.com/orgs/json-schema-org/discussions/671) which lists features that need to be put into the proposal process.
+
+## Context and Problem Statement
+
+In creating a stable spec release, it's important that all included features are well-developed and their behaviors are finalized.  As such, the following features will be extracted before the stable release.
+
+- `$vocabulary` - This feature is still in development.
+- Output formats - This feature is being extracted to its own spec anyway.  (See [PR](https://github.com/json-schema-org/json-schema-spec/pull/1429) for more info and link to discussion.)
+- `propertyDependencies` - This feature is not technically in the spec and should go through the proposal process.
+
+## Decision Drivers <!-- optional -->
+
+We can't publish a stable spec that contains unstable features.  Thus we need to remove them.
+
+## Considered Options
+
+1. Extract unfinished features and put them through the proposal process.
+2. Complete the features before releasing the spec.
+
+## Decision Outcome
+
+Chosen option: Extract unfinished features and put them through the proposal process.
+
+This allows us to release a stable version of the specification while continuing to develop these features.
+
+### Positive Consequences <!-- optional -->
+
+* We can release a spec earlier that fulfills our users' needs.
+* We can continue to develop these features.
+
+### Negative Consequences <!-- optional -->
+
+* These feature will be considered experimental until their proposals are accepted.  This may be a hindrance to adoption.
+
+## Pros and Cons of the Options <!-- optional -->
+
+### Extract unfinished features and put them through the proposal process
+
+* Good, because we can release a spec earlier that fulfills our users' needs.
+* Good, because we can continue to develop these features.
+* Bad, because these features are experimental now.
+
+### Complete the features before releasing the spec
+
+* Good, because the features will be ready to use when the spec releases.
+* Bad, because the spec may not release if we can't finalize the features.