For transmog in WoW Forever, Transmog Loot Helper has a Forever version and marks your items by whether you've collected them. Others are listed below with their status when we wrote this, so check each page for the latest. We haven't tried them in game. Our sister site reports that the Enhanced preset has transmog on and the Classic preset has it off, so these won't do much on Classic.
| Add-on | What it says it does | Forever when written |
|---|---|---|
| Transmog Loot Helper | Icons on items showing collection status | Ships a Forever file |
 Narcissus |
Character screen and photo mode | Ships a Forever file |
| Collectionator | Auction House tab for uncollected transmogs, pets, toys, mounts and recipes on your alts | Not confirmed |
| PeaversIconSearch | Item search box in the outfit icon picker | No Forever file |
Most of these were built with other versions of the game in mind, so what they track in Forever depends on how closely its collections match. Test on a few items first, especially before selling or deleting anything based on an icon.
Showing off the result
Narcissus has a photo mode, if you want a picture of your set.
More collecting add-ons are in the collections category.