As committed in bbe0a81d, pg_dump from a pre-v14 server effectively
acts as though you'd said --no-toast-compression.  I think the right
thing is for it to act as though default_toast_compression is set to
"pglz", instead, so that the tables' toast compression behavior is
preserved.  You can always get the other behavior, if you want that,
by giving the switch.

Discussion: https://postgr.es/m/1112852.1616609702@sss.pgh.pa.us