If I evaluate the same setq and define the same tasks, my result withe org-agenda tasks list are:

Global list of TODO items of type: ALL
Press ‘N r’ (e.g. ‘0 r’) to search again: (0)[A$
                      (1)TODO (2)DONE
  test:       TODO test1
  test:       TODO test2
  test:       TODO test3
  test:       TODO test4

I tested at this moment with Org version 9.5.2 on Emacs version 27.2

Le 5 avril 2022 07:44:25 GMT+02:00, Ihor Radchenko <yantar92@gmail.com> a écrit :
>Sébastien Gendre <seb@k-7.ch> writes:
>
>> I've tested it, but the tasks with no deadlines are still on top of the list.
>
>I am unable to reproduce on my side using latest stable Org.
>I used the following example org file:
>
>----
>* TODO test1
>* TODO test2
>* TODO test3
>DEADLINE: <2022-04-04 Mon>
>* TODO test4
>DEADLINE: <2022-04-06 Wed>
>------
>
>(setq org-agenda-sorting-strategy '((agenda deadline-down time-up habit-up priority-down timestamp-down category-keep)
>				    (todo deadline-up priority-down category-keep)
>				    (tags priority-down category-keep)
>				    (search category-keep)))
>
>The todo agenda buffer looks like:
>
>Global list of TODO items of type: ALL
>Press ‘N r’ (e.g. ‘0 r’) to search again: (0)[ALL] (1)TODO (2)DONE
>  bug:        TODO test3
>  bug:        TODO test4
>  bug:        TODO test1
>  bug:        TODO test2
>
>
>Best,
>Ihor