Formatting external HD for Monterey

I need to format an external hard disk for backup / Time Machine purposes.
I understand that starting with Big Sur, Mac OS Extended (Journaled) is no longer the default when using Time Machine and that I should opt for APFS.

I also have read that an external HD formatted in APFS will not mount on a pre Big Sur system.
I have a couple of old Macs limited to High Sierra. Thinking of keeping my options open in that
maybe some time in the future I’ll want to connect the HD to the older Macs, I should format the HD
in Mac OS Extended (Journaled).

Basically, is there any harm in formatting the external HD in Mac OS Extended (Journaled) for Monterey 12.2.1.?

Many thanks.

Here’s what Carbon Copy Cloner recommends to its users.

Preparing a disk for a backup or restore | Carbon Copy Cloner | Bombich Software

Isn’t it the case that you can keep going with your Time Machine HFS+ (TMH) if you had one before but if you’re setting up a new one it has to be using APFS (TMA)? One way or the other, I wouldn’t use TMH today if it’s possible to use TMA. Modern operating systems create such large number of files that after 2-3 years (max) the TMH will likely fall apart (due to the sheer number of hard links).

High Sierra supports APFS so there shouldn’t be a problem with accessing drives there. Time Machine on High Sierra doesn’t (likely, I haven’t tested it myself) support TMA backups but they should still be readable via Finder.

If you have some Macs that don’t even support High Sierra, then maybe file sharing via LAN would be a solution?

My understanding is that HFS+ will continue to work and when you initialize a new disc for TM it will be initialized APFS (pretty much as you said).

I don’t think this is anything to worry about because of HFS+’s directory hard link feature.

I bought a HD that was formatted with FAT32. I know this is the Windows file system, but I thought I’d give it a try. It was working fine, until I tried copying a file that was 6 GBs. Yup, FAT32 can’t store anything larger than 4 GB. Luckily I didn’t have too much on the drive, so I moved the contents to my laptop and reformatted with APFS. I know this doesn’t answer your question about HFS+, but I’d be wary of using a filesystem from a bygone era. It will probably have some limitations you didn’t expect.