I haven't heard of anyone being able to "force" an unsupported OS version on Apples mobile devices.
In theory it would prob be possible but the question is if its a good idea. Apples mobile devices are more locked down than the Macs (where its possible to install MacOS with 3rd party tools).
The situation with the iPad6 vs iPad7 is similar to the iPad pro 10.5. Its not supported but would prob run alright with iOS 18.
Havin said that, there might be loads of reasons why Apple have choosen to dump support for these devices.
Speed might be one but there might be a lot more that we don't know about.