The Old English representation, inclusive of the long S, is consistent with the 1847 source material, as you can see here: https://archive.org/details/firstsixbooksofe00byrn/page/n21/...

The first printed copy, of 1482, was in Latin: https://www.loc.gov/item/2021667076/

The Greek (Ελληνικά) representation is presumably consistent with Euclid's original manuscript, e.g. as represented in this copy, of 888: https://historyofinformation.com/detail.php?id=208

Sergey Slyusarev re-typeset it!

https://www.amazon.com/gp/aw/d/1684932289

There are a bunch of people reprinting it, because it's easy to just reprint scans, but this copy actually re-typeset it.

If you already know the subject the old typesetting is bad enough but you really don't want to try teaching someone who's never been exposed to Euclid from it, or they'll be wondering why it spends so much time talking about furfaces.

There is a menu from the top bar to change the "Modern English."

You get used to it very quickly. If you're actually interested, give it five minutes and you'll read it like normal.

If you're not interested, the long s and the ligatures are not the problem.

> in some ancient version of english with characters that I've never seen

There are really only 3, or if you want to stretch it 5, English characters that you might not be familiar with as characters: þ, ƿ, ð, æ, and ȝ. And the last of those isn't even present in Old English; it's a Middle English thing.

If you were reading something in Old English, the use of ƿ wouldn't really be an issue - the issue would be that nothing made any sense. Recognizing the characters used, or not, is irrelevant.

As you note, the recognition issue is an issue of fonts and not of alphabets. Compare wikipedia's sample of blackletter: https://commons.wikimedia.org/wiki/File:Calligraphy.malmesbu...