A couple of months ago I noticed there are considerable number of i18n
messages in mediawiki core that are not used in the code, it can happen
because you remove the code but you forget to remove the associated i18n
messages. This is not good as it puts unneeded work for our volunteer
translators. I started https://phabricator.wikimedia.org/T224429 and I
would like anyone who has done some code in core to take a look at the
lists there and take action if they know what can be done for specific
group of i18n messages.
Sometimes it's not super easy because the i18n key might get lego'ed or
sometimes the code is moved to skins/extensions (like 'watchthispage' )
which means the i18n should be moved to extensions/skins first and then
removed from core.