Currently, c. 20 different alphabets are wire into Cite on various wikis. During T377705: Investigation: compile a full list of custom footnote markers used on Wikimedia sites we discovered that CLDR data is available with many more alphabets. Supporting these alphabets for footnote numbering / lettering would improve localization and should be relatively easy to accomplish. This also fits well with the outcome of T370879: Investigation: How to best support custom footnote marker groups in VE, Parsoid, and read mode?: both investigations show that a simple "a…z, aa, ab…" pattern has proven acceptable so far, so we can deprecate the legacy symbol customization in favor of programmatic generation from known alphabets.

Backlink customization seems to also fit well with this approach, and may become the default depending on the outcome of T370882: Investigation: Should backlink numbering defaults be changed, eg. to alphabetic?.