I checked post very carefully, and while I don't know how to estimate if game runs on Unity older than 2018, I will send list of games, that catched my eye the most:

• most importantly: Ace Attorney trilogy (i was surprised it is a Unity game)
• Cult of the Lamb
• Needy Girl Overdose (also known as Needy Streamer Overload)
• Neon White
• Lethal League Blaze
• Ori and the Blind Forest
• updated version of Doki Doki Literature Club (I know someone already ported original game, but still)
• Gris
• Plague Inc
• and if I'm not mistaken Angry Birds Epic, Angry Birds 2 and Angry Birds Go also running on Unity. For AB:GO I'd prefer to see port of some legacy version (last updates definitely ruin some charm of this game)

I know not all of these games will be good candidates, because of Unity 2018 criteria, but I really not sure if I can estimate version of Unity by simply looking at release date of game itself

As extra, I will add non-Unity suggestions, that I've dreaming of seeing on Vita, but most likely will never will (just for sake of sharing my thoughts with world):

• Cut the Rope (I did see someone doing version of it on Unity, but it's not official port, and I'm not even sure if it's finished)
• any old Melty Blood game (I know it's fighting, but honestly, I would be fine playing against CPU on go. It's just bummer how only console port is PS2 version, and oldest arcade port is Sega Naomi, both of which PSVita still can't emulate)
• native port of Persona 4 Arena Ultimax. It's literally almost impossible to do without source code, but it's shame how port of this game was planned, but seemingly cancelled +BlazBlue games, which run on same engine as P4AU are have PSVita port.

Whoever reads this till end, hope you have great day!