I can’t back up the data in a PWA.

If you store notes on disk, which is what I do, you can back them up with any file backup tool.

I store my notes (encrypted with a password) on OneDrive.

See https://edna.arslexis.io/help-win#storing-notes-on-disk

I even have a settings option for a daily backup which exports all notes into a single .zip file in `backups` sub-directory. Not sure if it makes much sense, but it's there.

For me, I like that HeyNote auto saves to buffer.txt and I can have a crib job auto commit that to git at regular intervals.

You can do that with Edna too.

Store notes on disk (https://edna.arslexis.io/help-win#storing-notes-on-disk) and you can auto commit the whole directory with git.

Personally I store my notes (encrypted with a password) on directory managed with OneDrive.

Edna also auto-saves files.

not everyone has 24-hour online access

PWAs don't need online access. They only need initial access to download them.

Same with plain old websites if they're made to run offline (e.g. https://devdocs.io/offline).