3GPP 21.900 v20.0.0 — the document's own text
4.8 "Closing" of specifications
Taught in 3. What a Release is, and what freezing one costs (How standards actually get made, in depth), 4. The change request and the release (How standards actually get made, overview), 3. One idea, from proposal to published text (How standards actually get made, quick start).
A TSG may decide that a specification of a certain Release will no longer be maintained that it may be considered "closed". That is, no further Change Requests to the specification of this Release shall be considered. The specification of the Release remains available (e.g., for referencing reasons), but no further Change Requests shall be produced, even corrective ones to align with the equivalent specification of a subsequent Release.
All specifications of a Release will be closed when the TSGs decide that the Release is no longer to be maintained (i.e., the Release is closed). A specification of a Release can only be closed if the specification(s) for previous Releases are closed. At the same time, higher major versions of the specification may be under development (i.e. for a later Release).