(1)
Schenk was a unanimous case. That Holmes joined in the wartime anti-free-speech frenzy is less surprising than that he quickly began to disassociate himself from it in his
Abrams dissent.
https://www.law.cornell.edu/supremecourt/text/250/616 Not to mention later cases like his dissent in
Gitlow,
https://caselaw.findlaw.com/us-supreme-court/268/652.html his joining Brandeis's famous concurrence in
Whitney, etc.
https://caselaw.findlaw.com/us-supreme-court/274/357.html Taking his record as a whole, Holmes was definitely one of the more pro-free-speech justices of his time. (One should also note that he took a pro-civil-liberties position on some non-free-speech-related issues like wiretapping as in his
Olmstead dissent.)
(2) On
Buck v. Bell: eugenics was not really a left vs. right issues in those days--note that Brandeis joined in Holmes' opinion, and that the only dissenter was the very conservative Butler (which may have had something to do with his Catholicism--but it is hard to know Butler's rationale since he merely noted his dissent without writing an opinion).
(3) In general, in the 1930's Supreme Court justices were classified as liberal vs. conservative more than anything else on their willingness to sustain economic regulation laws (allegedly) passed for the benefit of the disadvantaged--and Holmes must also be considered a "liberal" in that sense, even though personally he thought much of that legislation unwise.
So, yes,
as the word "liberal" was applied to Supreme Court justices in the 1930's, Holmes has to be considered a liberal, however dubious it may be to so classify him philosophically.