Unicode Locales Stability Policy
Unicode CLDR Repository Stability
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.
Unicode LDML Specification Stability
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.