Refactor the existing thumbnailer code is core to split it out as a separate "thumbnailer service", written in PHP.
@cscott MediaWiki now supports deferring the thumbnail/image scaling effort to a separate service (as of rMWa9213ccb4c77: Add ability to proxy thumbnail requests to a service). Although even before this, one could set up rewrites to bypass MediaWiki entirely for thumbnail urls.
For Wikimedia production, this has been done, by using Thumbor (see blog post), and while we're continuing to maintain the MediaWiki implementation for smaller installs and for custom API methods, it's unlikely Wikimedia would dedicate resources to a dedicated PHP-based service for this (in the short term, anyway).
I'm just checking if you're still interested in having this task open? I imagine as part of the long-term Librarization effort, it might get done regardless, but we don't need this task for that.
Is the task about turning the thumbnailer into a library, or turning it into a service (as in MediaWikiServices), or turning it into a standalone service like Parsoid etc? I doubt the first is feasible (it needs to read the the database, extensions hook into it etc). T66214: Define an official thumb API Would turn it into a REST API (so sort of a service from an external POV although still running on top of MediaWiki).
I think this was an idea before the thumbor project started, in order to have a thumbnail service that didn't need to have access to the databases, for instance.
I don't think this ticket is valid anymore, so I'm going to decline it.