It was actually ſ+z, which is easier to see if you look at it in Fraktur, and is literally the name of the character. I have long been puzzled by how its romanization became „ss“. In older writing ‘s’ was just the terminal form of ‘ſ’ — if you needed two you just wrote ſſ, same as in English into the early 19th century.
Other ligatures like tz and ch did not survive the jump into Roman letterforms, only ß for some reason.
By the way the now obsolete dual letterforms of s (ſ/s) was inherited from Greek. We’re lucky it was still in use in Leibniz’s time or calculus notation would be more confusing!
Not only the romanization, also the capital form in German is SS.
The ß sticks around in German because it prevents some ambiguities. For example Busse and Buße are different words with different pronunciations.
Basically German orthography backed itself into a corner because of two rules: a vowel before a doubled consonant is short, and an s before a vowel is pronounced /z/. This leaves you with no clear way to write a long vowel followed by an /s/ sound followed by a vowel: VssV would make the first vowel short, and VsV would keep the first vowel long but the consonant would be pronounced as /z/. The ß solves this problem.
sz is officially still an accepted substitution for ß though. It's basically never used, except if you capitalize a word with ß and substituting ss would lead to confusion (if both words exist and would make sense). Of course since 2017 the upper case ẞ is also an option, but nobody wants to use that either.
There were ligatures for both ss and sz. The name for the sz survived for both. The modern eszett glyph usually looks like an ss ligature (long s + short s).
> The modern eszett glyph usually looks like an ss ligature (long s + short s).
It is literally a ligature of the long s and z. This is unsurprisingly obvious in Fraktur, given the name, but you can see it in the Roman form ß: the upper concavity of ‚s‘ opens to the right while the upper concavity of ‚z‘ opens to the left (the bottom of Fraktur Z in both cases had/has a tail like g which is how you get the bottom part of the shape).
I still have a few books printed in Fraktur — each time I pick one up (which is not frequently!) it’s is very hard to read at first, but after a few pages goes back to being easy. Since this phenomenon takes a little “priming” it’s annoying to encounter a sentence or phrase of Fraktur on a shop sign or other short, random location.
Another commenter linked to an explanation of why a vowel shift caused “sz” to reflect “ss”. The character form and name are fossils (something I wish German retained more of — it’s one of the things I love about English).
Other ligatures like tz and ch did not survive the jump into Roman letterforms, only ß for some reason.
By the way the now obsolete dual letterforms of s (ſ/s) was inherited from Greek. We’re lucky it was still in use in Leibniz’s time or calculus notation would be more confusing!