I was playing around, checking my sanity re : dynamic scoping.  I'm probably rusty, but I could swear that on older emacsen, this sort of code would print 33 ten times. 

On emacs -Q with debian stable (28.2).
I try this

(progn
  (setq lexical-binding nil)

  (dotimes (ii 10)
    (defmacro mac ()
      `(message "%S" ,ii)
      (sit-for 0.1))
    (let ((old_ii ii))
      (setq ii 33)
      (mac)
      (setq ii old_ii)
      )))

Exits without error the first two times, although, strangely, i don't see any messages the second time. (I expected to see ten 33's).

(a) As in, the second time, it doesn't print 33 ten times, as we would expect.

(b) The third time is stranger. 
Let's evaluate it a third time! The third time is even weirder.

The third time, it complains
Debugger entered--Lisp error: (void-variable ii)
  (list 'message "%S" ii)
  (lambda nil (list 'message "%S" ii) (list 'print ii) (sit-for 0.1))()
  macroexpand((mac) nil)
  macroexp-macroexpand((mac) nil)
  macroexp--expand-all((mac))
  macroexp--all-forms(((setq ii 33) (mac) (setq ii old_ii)))
  macroexp--expand-all((let ((old_ii ii)) (setq ii 33) (mac) (setq ii old_ii)))

and so on.

The bug(?) was reproduced on 29.1 as well - by e1f on #emacs

(c) It gets even funnier. e1f changed ii to var, but the third time, it still complained about ii, with the above error message, and not about var.  I reproduced that as well.

Dave