ULS has as minimum requirement MediaWiki version 1.26

@Fomafix: Where is that defined? Do you refer to https://www.mediawiki.org/wiki/Extension:UniversalLanguageSelector ?

$:foo\> pwd
/home/foo/git/wikimedia/mediawiki-extensions/UniversalLanguageSelector
$:foo\> grep -r "1\.25" .
./UniversalLanguageSelector.php: die( 'Universal Language Selector extension requires MediaWiki 1.25 or later' );
$:foo\> grep -r "1\.26" .
./UniversalLanguageSelector.hooks.php: * Conditionally register jquery.18n (backwards copatbility for those on pre-MediaWiki 1.26).
./resources/js/ext.uls.interface.js: // Replace with mediawiki.storage when >= MW 1.26
./resources/js/ext.uls.interface.js: // Use oojs-ui-core only after MediaWiki 1.26 is no longer supported

Wondering if this could become an good first task / GCI task (if knowledge requirements get explained in the task description, plus links provided to resources that allow a new contributor to judge whether they can work on this)?

Yes, I referred https://www.mediawiki.org/wiki/Extension:UniversalLanguageSelector

Change 333368 had a related patch set uploaded (by Fomafix):
Use module 'mediawiki.storage' from core to access localStorage

https://gerrit.wikimedia.org/r/333368

Change 333369 had a related patch set uploaded (by Fomafix):
Remove module 'jquery.i18n'

https://gerrit.wikimedia.org/r/333369

Change 333369 merged by jenkins-bot:
Remove module 'jquery.i18n'

https://gerrit.wikimedia.org/r/333369

Change 333368 merged by jenkins-bot:
Use module 'mediawiki.storage' from core to access localStorage

https://gerrit.wikimedia.org/r/333368

In T153635#2886928, @Nikerabbit wrote:

./UniversalLanguageSelector.php: die( 'Universal Language Selector extension requires MediaWiki 1.25 or later' );

At most the version number could be updated or removed, but this is a check for extension registration mechanism, that cannot be removed yet.

I think it is acceptable to keep the current text here because all other extensions have the same text here.

./UniversalLanguageSelector.hooks.php: * Conditionally register jquery.18n (backwards copatbility for those on pre-MediaWiki 1.26).

Just removal of some code, after double checking that core 1.26 indeed bundless jquery.i18n.

Done in 1312b905f2d7f129b06f37eb7bf274a8b75d7403.

./resources/js/ext.uls.interface.js: // Replace with mediawiki.storage when >= MW 1.26

Bit more complex change.

Done in 3ec9a325a4d90655b25f9cf04c3e1ae1e8efc127.