How do I deauthorize if I no longer own any of the authorized Macs???

I've been a Mac user since the beginning. My first Mac had 512k or RAM in it. That's 512 kilobytes of RAM, not megabytes or gigabytes. Since that time I've had just about every model Macintosh there was. I'd use them, then replace them as the newer, more powerful computers were released. I've gone through a lot of Macs! Even now, I have two MacBook Airs, a 17" MacBook Pro (I keep on my boat for navigation), a 13" MacBook Pro, and a 2020 iMac (which replaced my 2012 iMac.)

I tried to open an audiobook I had downloaded onto my 2012 iMac, which had migrated to my 2020 iMac. I got the message saying I could only have 5 computer authorizations. I found a discussion here which said I needed to deauthorize either one or all of my computers. I could load iTunes or Music, then deauthorize under Accounts. The problem is, my new iMac does not have one of my authorizations. I got out three of my four laptop Macs and none of them has an authorization. (I doubt the fourth one would, either.) The last authorized computer was probably my 2012 iMac, but it's now probably on its way to China to be stripped for its valuable elements and minerals. (In other words, I threw it out because it had a cracked screen. I was holding out for all those long years because I kept waiting for a total redesign of the iMac. Eight years with the same computer!)

So, how do I deauthorize computers that are long gone?