I would double-check to make sure it truly is relocated. (Check the same page again.) Is it really now on boot-pool?
Then for good measure, create a checkpoint before proceeding:
sudo zpool checkpoint Pool
Check if it’s mounted:…
mount | grep "\.system" | grep boot-pool
…compared to:
mount | grep "\.system" | grep Pool
Are mounts still being shown for the old System Dataset on the pool “Pool”?
Then do a “dry-run” of destroying the old .system dataset:
sudo zfs destroy -nvr Pool/.system
Does it look safe to continue? If so, issue the destructive command:
sudo zfs destroy -vr Pool/.system
After a few minutes or hours, if you don’t bump into a serious problem, you can remove the checkpoint:
sudo zpool checkpoint -d Pool
If you do need to rewind back to the checkpoint? Then you’ll lose any work / data that was done after the checkpoint’s creation.