Does Big Sur offically support *existing* encrypted HFS+ drives?

I have seen reports on these forums that Monterey may no longer access encrypted HFS+ external drives and encrypted .dmg files, but I am not certain. I have not seen any reports regarding Big Sur having any issues.

Things have gotten complicated with later versions of macOS (especially 12.x Monterey) and the M1/M2 Macs with Apple Silicon. Not all external devices are compatible with Apple Silicon Macs. Plus some third party devices and drivers tend to produce compatibility issues with other devices especially if those other external devices are connected when connecting & using an external drive (especially common with macOS Monterey -- less of an issue with Big Sur but may still occur from time to time).

Try disconnecting all other external devices from the Mac to see if that resolves your problem. Make sure to connect the external drive directly to the Mac. If you need to use an adapter to connect the drive, then try using another adapter especially a different brand (definitely try the Apple branded adapter. Try connecting the drive to another USB port especially on the other side of the Mac if you are using a laptop.

You can try checking the health of those external drives by running DriveDx. You will need to install a special third party USB driver in order to attempt to access the external drive's health information. However, even with the special USB driver, some USB chipsets used by adapters, docks, or enclosures may not allow the necessary communication to access the drive's health information. For hard drives, any "Warning" or "Failing" notices indicate a worn out or failing drive respectively. If these are SSD drives, then the health reports will need to be manually interpreted to confirm if they indicate a failing SSD. You can post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper.

I hope you have a backup of the data on those encrypted drives.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Apple is focusing more on APFS volumes these days, so you could try using APFS on the external encrypted drives. I'm not a fan of APFS at the moment since it seems to become corrupted where First Aid is unable to repair it which requires erasing the entire drive & restoring from a backup. It is hard to say whether a data drive would suffer the same fate or if it is just the constant access of a boot drive which damages APFS volumes. Also, the APFS file system is not a good choice for a hard drive as it makes the drive work much harder. APFS was designed for SSDs.

I believe macOS Monterey now requires a Time Machine drive to be formatted as APFS.

FYI, if you don't enable journalling on the HFS+ volume, then Linux can write to the volume. Whether this is a good idea I don't know since I'm not sure how well supported HFS writing is on Linux.

If you need to share encrypted drives between different operating systems, you should check out Veracrypt.