Each release of the Unicode CLDR is a stable release and may be used as reference material or cited as a normative reference by other specifications. Each version, once published, is absolutely stable and will never change. Implementations may also apply CLDR Corrigenda to a release.
The UTS #35: Locale Data Markup Language (LDML) may be changed to add structure for new kinds of data or other features. Structural changes are always backwards-compatible. That is, previous files will continue to work. Deprecated elements remain and can be used, although their usage is strongly discouraged.
Structural changes that require non-trivial code for proper implementation, such as time zone fallback or alias mechanisms require the existence of at least a prototype implementation that demonstrates correct function according to the proposed specification.
The structure of Unicode locale identifiers is backwards compatible. No subsequent version of LDML will make changes to Unicode locale identifiers that are incompatible with a previous version of LDML. More specifically:
The Unicode locale identifiers are dependent on underlying standards such as BCP47 or the timezone database (TZDB). In the event that one of these underlying standards makes an incompatible change in a code, a Unicode locale identifier that used that code would also reflect the incompatibility in ways that are otherwise inconsistent with Unicode Locale Identifier Stability.
The Unicode Locale Identifier Stability policy does not prevent the following kinds of changes from occurring: