But there are some – e.g. Auszeit. And at the same time, we use ß in locations where it'd otherwise be ss. While it started as a ligature, its current usage doesn't fit that.
That doesn't mean it's not a ligature. If English still had a "th" ligature, the word "outhouse" wouldn't use it.
Interesting claim ... a similar compound word with a present-day ligature is "halfling." TextEdit renders that with an "fl" ligature, but should it not?
Is w then still a ligature of vv, and should vve remove it?
th in English wouldn't be a ligature, it would be a letter (thorn or eth).
Absolutely true, it's not really its own letter nor is it really a ligature. It's an unique feature of the german language, which is hopefully here to stay...