Does macOS notify users of DEP if previously enrolled with QuickAdd?

Depends if you assign it to a Jamf Prestage after adding them into ABM.

One option you can do is assign them to ABM as you would like, but do not assign them to a Jamf Prestage, which would prevent the DEP notification from showing, once they're assigned a prestage and see they weren't DEP enrolled it would prompt them. Now unless the user has the flexibility to re-image when they want without creating a ticket or any other intervention, then it may pose a problem if you don't know who or when they will reimage and the device is no set to a prestage.

we have had manual enrolled devices that are DEP eligible, we set up a policy to "re-enroll" them, nothing really changes, no apps are reinstalled, just profiles as we use DEPnotify for initial enrollment but does not retrigger for something like this.

now with Big Sur showing manual enrolled devices as supervised, I don't know if it would trigger them or not, have not tested that scenario.

We have a lot of machines that get UIE'ed that should have gone DEP. They do not get a DEP nag AFAIK. There was a certain OS version that did result in them getting nagged (specific versions of High Sierra IIRC) but Apple "fixed" that.

So you should be able to safely add them to a prestage without the user getting nagged, and the next time they get wiped they will do a DEP enrollment as expected.

Thanks, I think High Sierra was about the time I saw that, so that tracks. I haven't been able to reproduce that in testing recently.