In the example "Straße", the ß is, in fact, derived from an ancient ligature for sz. Old German fonts often had s as ſ, and z as ʒ. This ſʒ eventually became ß.

We (completely?) lost ſ and ʒ over the years, but ß was here to stay. Its usage changed heavily over time (replacing ss instead of sz), I think for the last time in the 90s (https://en.wikipedia.org/wiki/German_orthography_reform_of_1...), where we changed when to use ß and when ss.

So while we do replace ß with ss if we uppercase or have no ß available on the keyboard, no one would ever replace ß by sz (or even ſʒ) today, unless for artistic or traditional reasons.

Many people uppercase ß with lowercase ß or, for various reasons, an uppercase B. I have yet to see a real world example of an uppercase ẞ, it does not seem to exist outside of the internet. For example, "Straße" could be seen capitalized in the wild as STRAßE, STRASSE, STRABE, with Unicode it could also be STRAẞE. It would not be capitalized with sz (STRASZE) or even ſʒ (STRAſƷE – there is no uppercase ſ) – at least not in Germany. In Austria, sz seeems to be an option.

So, for most ligatures I would agree with you, but specifically ß is one of those ligatures I would call an outlier, at least in Germany.

P.S.: Maybe the ampersand (&), which is derived from ligatures of the latin "et", has sometimes similar problems, alhough on a different level, since it replaces a whole word. However, I have seen it being used as part of "etc.", as in "&c." (https://en.wiktionary.org/wiki/%26c.), so your point might also hold.

P.P.S.: I wonder why the uppercasing in the original post did not use ẞ, but I guess it is because of the rules in https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.... (link taken from the feed). The wikipedia entry says we adopted the capital ẞ in 2017 (but it is part of unicode since 2008). It also states that the replacement SZ should be used if the meaning would otherwise get lost (e.g. "in Maßen" vs. "in Massen" would both be "IN MASSEN" but mean either "in moderate amounts" or "in masses", forcing the first to be capitalized as MASZEN). I doubt any programming language or library handles this. I would not have even handled it myself in a manual setting, as it is such an extreme edge case. And I when I read it, I would stumble over it.