Just one minor one why you might not want to. HTTP{S}? handling objects understand the URL parameter syntax and do parsing for you, so when you write a request handler, you are dealing with structured dictionaries of request headers and URL parameters.

If you're relying on HTML forms being filled in to generate queries, the browser will always generate standard query parameters with a question mark and ampersands. But if you're making your own links, you can generate them however you like. Semi-colons should work fine, though you may need to write your one URL parsing functions.

I think some browsers display the query params muted compared to the part before (or maybe I’m confusing it with the behavior that the domain is highlighted to prevent phishing?). But technically, you can just parse the request url however you want. I also think it shouldn’t matter with regards to any caching layers inbetween.

You can use it if you want, but the browser doesn’t help you at all. It can’t automatically turn a form into a matrix URI, and it can’t do anything useful with relative matrix URIs.