Comment by Aloisius

I imagine if you choose to ignore the warning that iCloud syncing will cease to work unless you disable ADP, then at some point, the warning turns into an error and iCloud syncing will cease to work.

I can't imagine they'll cancel your iCloud+ subscription. ADP is not a feature of iCloud+ and iCloud+ has features beyond extra storage space. Nor can I imagine they'll delete your data preemptively as long as there's space to store it.

Hopefully they'll provide instructions on how to manually delete your iCloud data in case you don't want to use it any longer (I think you just turn off iCloud on all your devices).