if you can legally do your own app using the device, you can do so regardless of abandonment of the original app. if you cannot do so, abandonment of the manufactorerers app has no impact on this. just make sure they cannot not sue you. because on their website they talk about taking down third party things.
lol. i did not help you. i just wanted to point out that a company abandoning a thing does not protect against getting sued.
the warning on their website seemed rather specific, so it might just be allowed if you do not do those things in your app.
i mean, a lot of the old game consoles also have new things being made for them, and you can find those on itch. you can download new roms for gameboy and others.