We used to include canonical meta tag on /c/plugin page but (I think) this was regressed 4 months ago when we introduced “Default Topic List” category setting.
This should now be fixed, and a test case is added to prevent future regression.
https://github.com/discourse/discourse/commit/72c92b0f4e2c324a9902613c9464fc45bf7fc09b
Now all the category pages should have canonical meta tag except the Top category page (/c/plugin/l/top).