From a purely judicial perspective, slavery is explicitly abolished in the US Constitution, via amendment.

The approach that Thomas favors is "text, history, and tradition". The words - as written - taken in light of what the author(s) intended and what was happening at the time.

The approach that Thomas (and the court majority) favors is “do whatever I personally want, then hunt for a justification, however tortured.”

With modern medical knowledge, abortion is safe. Does that make an unwanted pregnancy forced labor?

https://www.thenation.com/article/society/abortion-ethics-ge...

Except for prisons

If you want to get really technical, except as punishment for a crime.

As best I can tell it would be entirely legal for the US government to sell someone sentenced to life in prison to a private party.

Well... we actually have a constitutional amendment directly banning slavery, and no such amendment for interracial marriages which are never mentioned constitutionally. It's easy to resolve from a purely constitutional perspective.