GitHub is a great example of mostly server-side generated HTML. Their output HTML is very stable: the structure rarely changes, is semantic, logical, and self-documenting. This has helped hundreds of browser extensions (e.g. Refined GitHub, ZenHub) and userscripts, including dozens of my own.

Of course, treating HTML as API/data exchange format is fragile and might be seen as hindering progress by some of the GitHub staff. However, it has been like that for many years and was beneficial for the community. Perhaps not that beneficial for GitHub, who would prefer integrations over the official API.

If GitHub's HTML changes to a dynamic React-powered div-soup, that might be the end of browser extensions and userscripts. And another reason for power users to flock to other platforms.

Edit: React does not necessarily mean div-soup, but I have seen too many React-powered div-soups to expect GitHub's HTML to stay the same.