In the late '90s Netscape was a niche browser with negligible 80% market share. The real and eternal XHTML enlightenment had begun a few years later, in the early 2000s and reached near eternal duration of seven years.
Also, https://jakearchibald.com/2023/against-self-closing-tags-in-... provides a broad perspective on the topic (but I guess it is very unlikely anyone reading this hasn't seen that article already).
Practically, using `<br />` in HTML with space was safe, like, forever, except for original W3C validator and Amaya. Using `<br/>` is safe since around 2002-2008 when Netscape was dying. In 2026, you can throw basically anything at current browsers and it will repair it to something meaningful, as per the living HTML spec. You can go `</br/r/r>`, if you are really into solidi, and it will work the same as `<br>`.
Disclosure: I also clearly see how having stupid simple "XML-like" syntactic rules would be beneficial in the grand scheme of things compared to what HTML became: memorising the "VOID" HTML elements by heart, and having to implement this in every HTML processing product clearly creates significant mental and processing overhead. But FMPoV, it's just one inconvenience we should begrudgingly accept at this point, rather than fight it.
(⁕) In reality, the way browsers treat `<br>` in the document flow is more like a text node than element node, but it's just an implementation detail orthogonal to this topic.