Impeachable offense is whatever the congress says is impeachable.
Rep. Alcee Hastings (d) was a federal judge and he was impeached and removed from office because of a scandal involving gold plated plumbing fixtures. Then he became Rep. Hastings.
If congress decided they did not like Gingsbergs eyeglass frames they could impeach her. More likely they will find an excuse, vote, and boot her out.
However any impeachment involves a requirement in intestinal fotitude.
I do think that relying on unratified treaties is not only leagally unsupportable, it is also morally repugnant and an insult to the profession they purport to hold in high regard.
I think these justices who voted for Europe over the USA are doing this as a desperation move because they see the leftist days as numbered.
Judge Hastings was convicted on two Articles of Impeachment: perjury and conspiracy to obtain a bribe.