The nation whose ambassador was expelled may choose to downgrade relations formally, even if the other side has not, as its own protest, of course.
On the other hand, expelling an ambassador is a rare event and usually one piece of a fairly extreme diplomatic protest that involves other elements, either simultaneously or in close temporal proximity (fully cutting off relations or even declaring war would not be that unusual as accompanying actions.)
I just watched something about this (I don't know the channel so I'm unclear on the accuracy here) - see the second half of the video https://www.youtube.com/watch?v=LT_0IxVKGsQ
Summary: Netanyahu threatened Australian Prime Minister Albanese, and Albanese capitulated quickly.
[1] https://apnews.com/article/australia-iran-antisemitism-attac...