Can Time Machine use HFS+ backup disk under Big Sur or Monterey?

TM may not let you go back in time after updating to Monterey however if you use shift-command-C it will show your Macintosh drive and you can then go back in time opening up that drive and finding your older files. I now always start a new drive with any major OS upgrade. With a new drive Monterey will make the TM format APFS no matter what you may have formatted it earlier.

Similar thread. Do I have this correct?

• my external Seagate backup drive is formatted as MacOS Extended (Journaled)
• Seagate contains TM backups 10.13.6 High Sierra from my old MB Air
• these backups will be unreadable to MBAir 12.1 Monterrey
• transferring them to a new external drive formatted as APFS won't help

I just don't want to keep trying to read / retrieve them.

Time Machine can use HFS+ - now more commonly called Mac OS Extended - but it was replaced about 2017 by APFS and it that format which is preferred. In answer to your question I have a Time Machine backups external drive in Mac OS Extended which is overdue for reformatting, but I have no problems with it for the moment.

Rather, it may be possible that the problems with your drive have less to do with the file format and more to do with corrupted data. If you are having problems with it and Time Machine is giving you error messages you probably don't have much choice but to reformat the drive and start your backups again, and to reformat it using APFS. Or failing that get a new drive and hang on to the old one for a bit until you are happy to reformat it.

You are correct but like noted above you can access those with shift-command-c in a round about way. I always recommend starting a new drive with any new major OS upgrade. Keep your old TM drive just in case and get a new drive and use with Monterey which will automatically be formatted APFS.

Thank you so much!