Are we talking about the app operationally day-to-day, or the app vis a vis the way it helps end users flash encrypted firmware images OTA to the ESP32 (like Chip alludes to in gp comment)?
If there are no more app updates, presumably there are no more firmware updates either, so the flashing part probably doesn’t matter (and you can probably flash the last published image to an authentic device by other ESP32-y means if you really need to).
As to the day-to-day: Pretty sure the hardware still records regular ol’ MIDI files to the SD card. The app seems incredibly well-thought-out and ergonomic but, at the end of the day, nonessential to the device’s core function.
Somebody could presumably build and sell their own “MIDI Recorder Loop Nonstop Device Digital Piano Upgraded New 100%,” but it wouldn't be a Jamcorder, you couldn’t flash Jamcorder firmware updates to it just by slapping an ESP32 in a case, and the official Jamcorder companion app would not talk to it. Which… fair enough, right?
For that matter it wouldn’t surprise me if somebody could write their own client for the real Jamcorder, if they felt so inclined… but with the official stack so absolutely perfect from a design perspective, why would they?
But even things like SDCards move on, we've already seen that with SDHC. I've had EOL kit in the past that just doesn't support it because it's that old, and thus EOL and doesn't support that standard.
Saying "It's just writing to an SD" is today