I had never seen this message before, but if you use certain advanced features then you have to get rid of your old devices.
The two break points are using Advanced Data Protection which means that everything in iCloud uses end to end encryption. Including all york photos and things. The big trade off is that if you lose password you’ve lost everything so thing about whether you really need it. It also means you can’t use iCloud.com anymore as well without authentication from one of your devices.
Note that mail, contacts and calendars are never protected. The big risk is that if you lose your recovery code or you can’t access your trusted contact email you are toast. The nice thing is that you can have multiple recovery contacts so for instance it might be a gmail account that you have just for this purpose.
Net, net if you want to keep using your old iPads for things then you can turn this off and be a little less safe.
So if you are ok with using these models later: