Thumbor 7 will remove support for Python 2, requiring Python 3.6+. Previous versions of Thumbor do not support Python 3. That means that Debian Buster is required (T216815).
There are a number of breaking changes in Thumbor 7 (https://github.com/thumbor/thumbor/releases/tag/7.0.0a5), mostly around replacing callbacks with async/await.
Thumbor 7 will also allow extensions to easily add new handlers, which may obsolete some of our custom code.