The issue was in a page that @Bift accidentally didn’t link to correctly (the first link), the issue is actually a Safari bug related to the use of a scale parallax.
As a useful troubleshooting technique, here’s what I did:
I duplicated the site to a scratch project, to be able to work on it without fear of losing work
I opened the site in the desktop safari responsive design mode, and verified the horizontal scrolling problem was there
I removed all pages except the page where the problem happens
I started removing layout blocks from the page and each time went back and forth to preview to see if the problem was still there
doing this I eventually found the element that, when removed, removed the horizontal scrolling
So doing this is a useful diagnostic technique, and might not lead to a fix, but anybody doing this beforehand can help us a lot.