I am curious about what carts use this, I have made another thread on best homebrew titles but I learnt that some devs make their own mappers?
Which games have these?

Thanks

If you mean custom mapper as in the brewer made it themselves for just that game, then Star Keeper did that.

If you mean a mapper that was developed for sale to homebrewers and not used by Nintendo during the life span, then many. Memblers makes the GT-Rom board. The Incident, Scramble, Spook-o'-Tron, and others used it. I think some of NES Lives boards are new mappers as well.

Edit: GT-Rom Board also for Candelabra Estoscerro, NEScape!, Rollie