If a caldav-synced client deletes tasks - those deletions get synced to Nextcloud server. There is a trashbin for caldav though (introduced with Nextcloud 22.0) which is enabled by default. After a configurable period of time (default: 30 days) items get deleted from the trashbin.
No, as far as I know Nextcloud doesn’t keep different revisions of calendars (except for the trashbin mentioned before). The activity app shows calendar changes though.
You could use calcardbackup. Have a look at the README in the repo for creating regular calendar/adressbook-backups (including tasks).