time machine local backups cannot be disabled anymore (was possible in sierra). these backups are going to a hidden volume and can be listed with the tmutil(8) command. it is not possible to delete all local snapshots with a single command either, the delete subcommand takes a single date...
for this reason i do only on-demand backups when i connect my external backup drive.
another worrying thing is that the time machine backup drives still use HFS+ because APFS does not support directory hard links. so now i am backing up APFS files to HFS+ "bit perfectly".
my worry is backing up from FS A to FS B where these file systems are not feature equal. just the fact that you cannot use APFS to backup APFS is worrying to me.
for this reason i do only on-demand backups when i connect my external backup drive.
another worrying thing is that the time machine backup drives still use HFS+ because APFS does not support directory hard links. so now i am backing up APFS files to HFS+ "bit perfectly".