MiSTer is fully open source project and will remain open.
This is my hobby and i'm getting fun to make hardware and software for this project. While software part just requires time, hardware requires money as well. To release a working add-on i have to make some test samples. Sometimes disaster happens and some equipment/device dies during experiments. Today i've lost my precious DE10-nano board. Couple days ago some other board has died during experiment. Some bought devices/components intended to be MiSTer companions turned to be useless. So i have to try many options before give advice to users.
I'm not a big fan of asking donation for hobby projects, but after several such incidents i've thought to make it. It's absolutely voluntary, you don't have to. But if you satisfied my work and would be glad to donate, then here is my paypal:paypal.me/FPGAMiSTer
This is my first experience with donations. If something wrong, then let me know (or teach me).