It's not really enabled right now, but my offsite backup is a combination of a Raspberry Pi 4B, a QNAP TL-D800C and a Tasmota WiFi power plug at a family member's place.

I SSH in to the always-on Pi over a VPN connection, send a command to the Tasmota to turn on the QNAP disk shelf, do a zfs send and once it finishes it shuts down the disk shelf again.

You could look into AWS Glacier or S3 Deep Storage tier. If you have 20TB stored that’s about $20/month(YMMV) which isn’t wonderful but that’s a lot of data so it’s understandable.

Being a cheapskate, if I can get something back or it’s not crucial it’s on a RAID array with snapshots, everything else is either encrypted Duplicati backups to Google Drive (Windows) or encrypted borg backups to Borgbase(Linux)

Borgbase is very reasonably priced and if you have a large storage space in GDrive due to having one of their other services it’s a good use of it.

I use Scaleway Glacier since I could actually afford to pull the data out, unlike S3.

How about good old offsite HDDs, tapes, etc. I guess it depends on the target. If it's family photos, you probably want something like this -- after all, if you get hit by a bus and stop paying the hosting bill for a couple of months, all that stuff could be gone.

Variations on the scheme include rotating media into safe deposit boxes at a bank, etc.

The amount of data I backup offsite is significantly less than 20 TB. Therefore, my answer to your question will probably not help you.

I store my offsite backups at rsync.net and in one of Hetzners Storage Boxes. For backups in general, I use Borg.

The real question is how much of that data is irreplaceable. While I hoard like most of you I only off-site backup the hand full of TBs I can't live without if there was a full system failure. It's not the perfect solution but most of my hoarded data isn't mission critical

EDIT: to answer your question though I use AWS glacier storage