There is a plug-in which does, i'm using it to keep notes about clients and worfkflow, i don't want to ' abuse' my Keywords for this. It's called Daisynotes and you can annotate folders, collections and smart collections:
Another handy plug-in is 'Big...
I'm with Johan on this. Don't save those derivates, just generate them again when needed.
To answer you question; there is a one situation this option is missing and that is when you are 'in' a smart collection. Could this be the case?
I did not know about this behaviour but LrC does it on my system as you described. When i type in more characters than fit in one line, the field is streched to a second line and so furher.
LrC 11.1 and Mac OS 11.6.2
I'm not familiar with this plugin otherwise i would share my experiences. I think it does not delete the duplicates it finds but only marks them so you can decide yourself what to do with them.
Welcome to the forum by the way!