Repository: nodejs/node
Pull request: doc: clarify that features cannot be both experimental and deprecated

As discussed in the TSC meeting. The rationale is that either an experimental feature can be called experimental only if breaking changes (incl removal) would not break the ecosystem; if removing an experimental feature must go through a deprecation cycle, it is de facto no-longer experimental.

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Changed files:
filename: doc/contributing/collaborator-guide.md
patch: @@ -387,6 +387,12 @@ undergo deprecation. The exceptions to this rule are:
 * Changes to errors thrown by dependencies of Node.js, such as V8.
 * One-time exceptions granted by the TSC.
 
+Experimental and undocumented APIs are not considered stable, therefore are
+typically removed without a deprecation cycle. However, if such API has gotten
+some non-trivial adoption in the ecosystem, it (or a subset of it) can undergo
+deprecation – at which point, changes to that API (or at least, its deprecated
+subset) should follow [semantic versioning][] rules.
+
 For more information, see [Deprecations](#deprecations).
 
 #### Breaking changes to internal elements
@@ -526,8 +532,8 @@ the three Deprecation levels. Documentation-Only Deprecations can land in a
 minor release. They can not change to a Runtime Deprecation until the next major
 release.
 
-No API can change to End-of-Life without going through a Runtime Deprecation
-cycle. There is no rule that deprecated code must progress to End-of-Life.
+No deprecated APIs can change to End-of-Life without going through a Runtime
+Deprecation cycle. There is no rule that deprecated code must progress to End-of-Life.
 Documentation-Only and Runtime Deprecations can remain in place for an unlimited
 duration.
 
@@ -1059,6 +1065,7 @@ need to be attached anymore, as only important bugfixes will be included.
 [node-core-utils-issues]: https://github.com/nodejs/node-core-utils/issues
 [ppc]: https://github.com/orgs/nodejs/teams/platform-ppc
 [s390]: https://github.com/orgs/nodejs/teams/platform-s390
+[semantic versioning]: https://semver.org/
 [smartos]: https://github.com/orgs/nodejs/teams/platform-smartos
 [unreliable tests]: https://github.com/nodejs/node/issues?q=is%3Aopen+is%3Aissue+label%3A%22CI+%2F+flaky+test%22
 [windows]: https://github.com/orgs/nodejs/teams/platform-windows