I'm asking about security, not performance or anything else. How does 11 make old hardware less secure than 10, assuming it's only new features that cannot work?
Performance is important, as you don't want to have a dysfunctional system, but, typically, you have to disable important security mechanisms to force the install.
The same security mechanisms are available in new Windows 10 builds, but it does not enable them automatically because of said ancient machines.
In particular certain security issue mitigations are terrible performance on older CPUs (think easily up to 20% performance loss).
If your machine is ancient enough, it won't even get Core Security.
You lose on both sides.