The list I know so far (armor/weapon styles that we still cannot craft today & not going to in near future & can't be found in any costumes & Not some important NPC exclusive gear):
1.Old Huntsman style: Medium helmet and arm cops. Found only in hawk's eye armor set
https://en.uesp.net/wiki/Online:Hawk's_Eye
2.Ash'abah gear: Seems like it at least has a body gear and there is a helm gear during one of the quest
https://en.uesp.net/wiki/File:ON-npc-Talia_at-Marimah.jpg
*Seems like they are a kind of dyed ashlander style
3.Yokudan 2 other weapon variations: Only 1 kind of yokudan weapon set appear in your outfit station, while there are 3 variations for each weapon (those in outfit station even have names like yokudan shield 1 etc)
https://en.uesp.net/wiki/Online:Yokudan_Style
4.Old Ebon style: a full heavy set armor, found only in ebony armory set (Crypt of Hearts)
5.Lich style: a full light armor set + a staff, found only in shroud of the lich set (Crypt of Hearts)
6.Reach winter style: a full armor set (no weapons), drop in three sets (2 in maelstorm arena, the other is wrothgar overland)
*There are 3 costumes that are this style
7.Refabricated style: A full set, drop in hall of fabrication (seriously tho
@ZOS_GinaBruno I can understand other styles but why we still can't get this one

)
8.Savior's Hide style: a full medium armor set, drop in Hircine's Venner set (Selene's Web
Some single piece of style:
Titanborn's helmet, Golden Saint's shield, mannimarco staff
Most of these's appearance can be see here
https://en.uesp.net/wiki/Online:Styles
Feel free to add the ones I missed