It looks like the code for Thumbor extensions currently lives in rTHMBREXT Thumbor Plugins.
Should be moved to Gerrit as per T191182.
- Request a repository on Gerrit
- Make rTHMBREXT to observe changes from Gerrit https://gerrit.wikimedia.org/r/#/admin/projects/operations/software/thumbor-plugins
- Potentially set up CI for the new Gerrit repo.