Thinking about this question, I wanted to look at historical data. Then I noticed that while Celestrak does provide Archived TLE data for Salyut 7, Mir, Kvant-1, Kvant-2, Kristallm, Spektr, Priroda, and ISS (Zarya), it does not provide data for the recent Tiangong-1 and Tiangong-2 stations, nor for Skylab which was in orbit from 1973 to 1979.
Why not?
I'm wondering if there are any governing international agreements, policies, courtesies or considerations involved, or if this is just random happenstance.
edit: For this particular question which has remained unanswered for a month, I'll be interested in thoughtful speculation, if that's what it takes to address this.