(require 'url-http)

(require 'url-parse)

(defun url-retrieve-thread (url)

"Wrapper around url-http that download `URL' without blocking main thread."

(let* ((parsed-url (if (url-p url) url (url-generic-parse-url url)))

(mut (make-mutex))

(cond-var (make-condition-variable mut))

(buf nil)

(cb (lambda (&rest _) (setq buf (current-buffer)) (with-mutex mut (condition-notify cond-var)) )) )

(url-http parsed-url cb nil)

(with-mutex mut

(condition-wait cond-var) )

buf))

Idea was to call it from thread. In most trivial case:
(make-thread (apply 'url-retrieve-thread "http://example.com" nil))

It makes Emacs unresponsive (even C-g does not work).
I tested it on Emacs 28.2 and once of recent development versions (emacs-31.0.50)

Happy Hacking!