I recently decided to replay Morrowind but with only official expansions, and add-ons but when I get to some of them mainly Lefemm armors something made me think if they are actually part of Elder scrolls canon or are they similar to the whole space cores from Skyrim?

The space core obviously isnt canon. It was made by Valve to celebrate Skyrim being the first Workshop compatible game (Aside from Team Fortress).

Unless otherwise states, you can safely assume anything Bethesda made is considered official canon.

Yes, the Morrowind add-ons are accepted as canon.