It's not just the old console CPUs that aren't being made anymore but also the chips in the cartridges themselves.I have lots of Nintendo handhelds, and I love each one of them. I also know that hardware isn't being made any more. Those custom CPUs that Nintendo designed in partnership with chip makers aren't rolling off the lines. However many of them in the world there are right now is the highest number of them that will ever be going forward.
Those MBC1/MBC3/MBC5 chips used in GB/GBC carts aren't being made anymore so some flashcart manufacturers have developed work-arounds that use tiny FPGAs in new carts to emulate the MBC chips themselves, i.e. BennVenn & Modretro.
Others are scavenging them from junk carts like Mary-Kate and Ashley's Daily Planner which has an MBC-3 in it.
HDR and InsideGadgets are re-using MBC-3 chips for their GBC carts w/RTC.
Then there's the cheap reflashable junkcarts you can find on Ali with the notorious black "blob" on the PCB instead of a real chip. Those can be finicky at times and I stopped using them after a while.
But even with a FPGA-based cartridge one can run into issues with the firmware used on the cart itself.
Case in point, I have three BennVenn v4 flashcarts. Two are on the newest firmware and work in everything - FPGBC, Chromatic, GBC, Analogue Pocket, & GBA/SP. One is a v4 on older firmware that crashes after you move Pokemon to/from PC storage but only on FPGBC & Analogue Pocket. Works just fine in real GBC/GBA hardware *and* for some odd reason also works just fine on the Chromatic itself.
I've got Pokemon Ambrosia flashed to it now and only using it with my GBC & Chromatic. So far so good on the Chromatic.











