OpenScore: whether to replace "ß" with "ss" in German language text?

I would not change it. It was the correct spelling at the time, so should be kept.
In modern musing just follow the rules, use ß and ss where the rules say so, like the pre-1996 "daß" is "dass" nowadays, but "Straße" keeps the 'ß'. Unless you're in Switzerland... AFAIK they don't have that 'ß' on their keyboards, so always transcribe it as 'ss'

No, that would look weird and Swiss.

We even got an uppercase Eszett ß approved into Unicode recently, although it was never part of formal orthography it still saw use, mostly in titles.

I’d much prefer keeping to the original spelling, even if even older than the “old” spelling (if I read “Blüthenbaum” instead of “Blütenbaum” in a Brahms piece, so be it).

Rationale: if some user wishes to adapt it to de-{DE,AT,CH}-{1901,1996} spelling (yes, we have three per-country versions of two major releases, both of which are currently in use (many reject the reform/“deform” of 1996, which had about seven or eight patchlevens since 1996 by the way…), of our orthography) they can do that themselves.

If I compare it with some vocal scores of major works from the music publisher "Edition Peters" (they don't have an imprint, but I bought them 10 or 15 years ago), they didn't change "ß" to "ss". For another in Germany common music publisher "Bärensteiner" I don't own a score with a German libretto.

But: Although in this editions "ß" wasn't changed, I don't believe, they're using the ortography in time of Bach, Brahms and other (There were "orthography" reforms before 1996 (in 1876 and 1901)).

So for me it would make sense to left "ß", if the spelling of the source is the same as from the time of the composer, but it wouldn't make sense for me, if it's from a time maybe 50 years ago.