PAR2 (libpar2/par2cmdline) continues to see active (if sporadic) maintenance. The most recent tagged release was 2020-02-09 but there've been a handful of PR's merged since. [2]
PAR3 has a reference implementation and alpha spec (libpar3/par3cmdline) which is based around Blake3. [3]
OG PAR (libpar/parcmdline) is legit unmaintained; the last release was 21 years ago. [1]
[2]: https://github.com/Parchive/par2cmdline
par2 even has options for specifying level of redundancy. I've had good experience in recovering large corrupted files from an external drive - since then, I've incorporated it into the automated backups of my personal infrastructure.
Also check out https://github.com/Parchive/par2cmdline
I wish something like this was integrated into a modern and free archive format. RAR has it. RAR also got volume recovery: when creating split multi-volume archives you can also create recovery volumes - each recovery volume can replace any other one missing volume. It was a life-saver in floppy-disk times.
Look into par2 files (https://github.com/Parchive/par2cmdline).
Arrange your 'backup data' in some logical way, then generate par2 files for logical "groups" (i.e, per directory containing a set of files, etc.) with an amount of 'redundancy' you feel comfortable with.
You can then periodically run par2 scans of the static backups to both detect changes, and to repair the changes (provided the changes are not larger than the amount of redundancy you originally requested).
In addition, I sometimes have par2 files[2] on the DVD as well, though it's probably better to just have additional dvdisaster ECC instead.
It's a good idea to periodically go through all your old backups and transfer them to new media, or at least make sure the old media still works and start recovery on them right away if they don't.
[1] - http://dvdisaster.net/
* Store par2 parity information alongside your cloud storage and/or NAS: https://github.com/Parchive/par2cmdline
* Use a file system with strong data checks and repairability, basically meaning, ZFS with a sufficient redundant set up. "zpool scrub" can do wonders, and you can guarantee backups to a different pool are identical to the source.
When I burn data (including xz archives) on to DVD for archival storage, I use dvdisaster[2] for the same purpose.
I've tested both by damaging archives and scratching DVDs, and these tools work great for recovery. The amount of redundancy (with a tradeoff for space) is also tuneable for both.
[1] - https://github.com/Parchive/par2cmdline
[2] - http://dvdisaster.net/
The fork compiled for me this week, when the official 0.3 version on Sourceforge wouldn't. I vaguely remembered par3 being discussed, but couldn't find anything usable. And that's an example of why to be wary of new formats, I guess?
It's kind of like RAID for data, and removes the uncertainty that maybe ddrescue wouldn't get an I/O error and just receive and save bad data (it happens). It'd also be capable of repairing damage, up to the redundancy level par2 archives are created at (default is 5%, can go all the way to 100% -- which would mean you don't need any of the original data assuming all par2 files are not corrupt either (which it can also handle...)).
But I dogress. When you create archival packups you use must use error correcting systems. Par is a good place to start. https://multipar.eu/ https://github.com/Parchive/par2cmdline
[1] - https://github.com/Parchive/par2cmdline