There's a way. If u dont want updates, mess up your devices CSC code. For example. If u are on an ATT Device, install a verizon firmware using Patched Odin. The only caveat is that you have to use a firmware with similar bootloader version as u cant downgrade the bootloader. This would prevent the phone from updating as the CSC doesnt match. I did this with a friends Note8, it was a Verizon phone on Cricket wireless with Sprint software. It never updated until I installed XAA software on it, which is U1 Unlocked firmware.
Oh, you want the software update page to say "unsupported firmware"?
It's called "planned obsolescence" and the industry depends on it. They know exactly how far they can go and, if they went there now, they'd be out of business. I do NOT agree with this practice at all when they charge you $1,200.00, or more, for the latest $10.00 phone.
There is a MUCH better way to disable automatic system updates.
First you need to enable "Developer options" (please Google to find out how to do that), and then turn off "settings / developer options / Auto update system".
Those of us using US Unlocked versions of the S9 and S9+ in Australia and New Zealand have had to disable the most recent firmware update because it bricks our phones. The only fix is to revert to the previous (working) firmware version and disable auto updates.