How to hide files from find-file in emacs?

Redefine this function in your config (I recommend using :override advice).

(defun find-file-read-args (prompt mustmatch)
  (list (read-file-name prompt nil default-directory mustmatch)
    t))

Make it pass a PREDICATE function to read-file-name (see which). That function will perform your test.

(defun my-find-file-predicate (file)
  "Return nil to exclude FILE from completions."
  ...)
(define-advice find-file-read-args (:override (prompt mustmatch) my-predicate)
  "Make find-file' filter completions with my-find-file-predicate'.
This is advice for `find-file-read-args'.  Remove it with:
\(advice-remove \\='find-file-read-args \\='find-file-read-args@my-predicate)"
  (list (read-file-name prompt nil default-directory mustmatch
                        nil 'my-find-file-predicate)
        t))
;; (advice-remove 'find-file-read-args 'find-file-read-args@my-predicate)

You could alternatively let-bind completion-regexp-list around the call to read-file-name, but that's for positive rather than negative matches, so the regexp would be a lot gnarlier.

(defun my-find-file (file &optional wildcards)
  "Like find-file, but respect `dired-omit-files'."
  (interactive (list (read-file-name "Find file: "
                                     nil
                                     default-directory
                                     (confirm-nonexistent-file-or-buffer)
                                     nil
                                     (lambda (f) (not (string-match-p dired-omit-files f))))
                     t))
  (find-file file wildcards))
(global-set-key (kbd "M-f") #'my-find-file) ; Bind M-f (A-f) to the command.