Discourse has been really nice for about a decade. I'd go as far to say that the remaining challenges are not technical.
Nothing beats vBulletin or IPV...
And it works well that's why lots of of big players use it(KDE, Nvidia...) even Microsoft for the Flight simulator forum but it is true that you have to get to know how it works : the go to today timeline button for example, see comment in context etc. Once you are used to it you enjoy it and see how it pushes people to read the whole discussion before answering unlike reddit or here where people may tend to place their focus on threads with lots of comments & ignore others.
Because of a load-bearing CSS attribute, as I understand it.
So, it's hard to consider it a web-standards supportive platform anymore.
Or, at least, it's a web platform with a technical challenge of not being interactive (so users can post and interact) from web standards supporting devices but lacking whatever HTML standards were introduced since as recently as 2021.
I'd call it a technical challenge. Literally the CSS language framework / build process is just not that flexible.
Semantic design development process became separated from semantic HTML serving somewhere along the way.
Maybe that's fine and quite good for 99% of uses. But I see this one as a glaring technical question mark.
Bringing it back to the titular point in the OP, the "crappy forums" do not seem to (cause some users to) suffer from this problem.
[0] https://meta.discourse.org/t/dropping-ios-15-other-old-brows...
The phones that can't upgrade past iOS 15 are over ten years old. What is the current iOS 15 user base? It's probably more cost-effective for Discourse to gift affected users a compatible device than to keep old code in place.
Let's just make something better, please, and that doesn't require JavaScript as we are at it.