https://meta.wikimedia.org/wiki/Logo#Temporary_logo_variants says "Interface administrators should not override the logo with a temporary logo by editing MediaWiki:Common.css of their wiki, as this would make web browsers download both logos and would increase traffic (see T256295#6496718 for technical details). Instead, follow the instructions to request a logo change." The process involves setting up a Python tool + doing a mediawiki-config deployment. That's complicated for most editors and an unnecessary bottleneck. It's also too limited - typically one would want, at the very least, to add a link to some announcement to an anniversary logo.

The other available options are also not great - a gadget or CSS change requires some programming skills and advanced permissions, and uploading the logo file locally results in unpredictable timing and doesn't work with modern skins.

Anniversary logo configuration should be moved on-wiki so that any administrator can easily do it.

Community feedback from MediaWiki:

Idea: allow changing logo locally for celebrations etc
Right now, a Phabricator ticket is needed, which means that the community needs to be well-planned when asking for it. I am aware that this is a feature that might be extra sensitive, so perhaps this one should only be changeable by interface admins. (Obviously, there might be some specs for allowable files on Commons, like size, ratio, format, etc. I am not asking for the community to be able to use any file.)