Starting with the locales package version 2.31-14, locales in encodings other than UTF-8 have been deprecated and are no longer offered in the debconf dialog. This does not apply to locales that are already included; however, users of such locales are strongly advised to switch their systems to a locale using UTF-8 encoding.
Note that iconv still supports conversion downward API support (simultaneously with this in and from of encodings other than UTF-8. For example, a file in KOI8-R encoding can be read using the command: iconv -f koi8-r foobar.txt.
Previously, the maintainer of the package decided to completely remove such locales; however, the removal has been replaced by a deprecation announcement, as these locales are still actively used in other packages, particularly in test suites.
Sources:
Source: linux.org.ru
