I believe there's a good amount of users using typescript-plugin-css-modules for, well, CSS modules. The ones using Sass will have to have sass dependency installed anyway in order for Sass to work in their project (correct me if I'm wrong).

Sass makes almost 50% of typescript-plugin-css-modules's install size:
https://packagephobia.com/result?p=typescript-plugin-css-modules
https://packagephobia.com/result?p=sass
so making it optional would significantly improve package install time. Especially since sass depends on chokidar which depends on fsevents which depends on (God forbid!) node-gyp.

Therefore, I think it would be worth considering making sass an optional peerDependency.