I am unable to locate the specific newsletter from Ask Woody as all my searches return a reference to single line registry edits, but a newsletter received in my email in the past month or so appears to provide what I'd consider an easy solution.
Windows 11 requires TPM in order for the OS to be automatically installed to the specific machine:
Trusted Platform Module (TPM) technology is designed to provide
hardware-based, security-related functions. A TPM chip is a secure
crypto-processor that is designed to carry out cryptographic
operations. The chip includes multiple physical security mechanisms to
make it tamper-resistant, and malicious software is unable to tamper
with the security functions of the TPM.
I've found a Tom's Hardware post that suggests one can create a work-around to install Windows 11 on a computer that would not be supported (officially?) by Microsoft, but since your objective (and mine) is to prevent an auto-upgrade, that's not the important part.
I was able to change my TPM support to disabled. Granted, it reduces the security of my machine, but it also prevents MS from pushing an operating system to my computer.
If your computer does not currently support TPM, you shouldn't get the upgrade. If it does, check your UEFI settings under Security to see if you have the ability to disable it.