I don't think it disappears - the copy I have will still be on my machine, and free to use as well. Unless they implemented something to remotely delete it?
Dosbox is a testament to that.
The original iOS version worked. Maybe don't update iOS if you want to continue using affinity's software?
Adobe CS2 is a highly-capable software suite that would happily run on today’s computers. I remember when Adobe shut down the license servers for CS2. They released a version that you didn’t need to activate to assure people that they would still be able to use the software they bought in the future. But then they got tired of hosting the download servers, so they stopped, and that was it.