After updating on Monterey, my Mac tells me that "Some system extensions aren't compatible with current versions of macOS or won't be compatible with a future version of macOS." But it doesn't tell me what it is.

This links below may send the user in the right direction in establishing which Kernel Extension maybe playing up.

About system extensions and macOS - Apple Support

Deprecated Kernel Extensions and System Extension Alternatives

If an app uses System Extension(s) of its own, they are embedded in its package, so there is no direct way to identify incompatible extensions.

macOS also has many Extensions of its own but none of them would be in question regarding compatibility.

You could start your investigation by considering the following:

• Identify any older apps that you have, especially if you know or suspect that they use extensions. In my experience most apps that use extensions notify you when you install or update them. Simply updating or reinstalling the app usually updates any extension it may use. (Audio & video apps often use system extensions.)