prestiging mons

I almost always used Furrets or other evolution fodder for that purpose, and only ever had a few dedicated prestigers that I didn't just transfer after I used them. So no, not really, at least not a large number of them.

I use about 15 of them as my gym defenders. High enough CP to hold for a while, low enough to get kicked out. Makes me my 50 coins.

Just transfer and evolve more if the game goes back to needing those...

Usually, I keep 1 or 2 high IVs of each mon so if it's needed again, you can always use what you have saved (like hatched Tangela's, etc...) or 1 or 2 of each if you keep a living dex.

Evolve fodder also can rebuild a prestige army quickly again so I wouldn't worry saving them.

I transfered mine. If they are ever needed again, I can just catch/evolve new ones.

Yeah, at least place them in gyms and then transfer them when they return with no health. Send them off to one last battle. I'm in the process of doing this (it's amazing how much space these guys were taking)

Nope. Ground them all into candies during the double everything event. It was the best way for them to go out.