For various reasons, sometimes fields need to be cleaned up/hidden within Marketo. This requires removing field references first, which ensures the field is not in use prior to hiding from the Maketo UI.
However, when a field is in use in the Smart List of an Email program AND that Email program has been sent, there appears to be no way to remove the field from the Smart List.
Of course, one can obviously delete the email program altogether, but this results in loss of any metrics, the email asset itself, etc.
I have engaged with Support, but meanwhile, I'm curious if anyone else ever dealt with this before.
I bet a lot of users are waiting for the day Marketo lets hiding a field without users having to remove all the references from the archived/completed email send programs/assets. There's a similar idea here on the community-ideas board if you'd like to upvote.
You used to be able to remove field references (I believe as recently as late 2020); however, as you have noticed, they made an update.
I dealt with something similar recently, and I believe, at this point in time, Support is the only one who can remove the field reference from a sent program.