Works after downgrading to PHP 8.0, deprecation-messages only with PHP 8.1. Fixed with updated code for the Requests-component in WordPress-core a few days ago – expected to be released with WordPress 6.2 then.
https://github.com/WordPress/wordpress-develop/pull/3732
I think/hope I was still on time now to check it out.
It didn’t perform *any* XHR for me. Actually it even failed loading a JS-file (some minify-thing) with a 500 Internal Server Error. I guess that might be relatd?
Seems to work for us with the browsers we tested so far, yes (Firefox, Opera, …) and logs are back to normal.
jQuery offers both alternatives – either giving a string with ampersands or giving the parameters separately as a JS-array.