Older MacOs not recognizing Time machine share MacOs High Sierra

Read the detail in "Disks you can use with Time machine" ( https://support.apple.com/en-gb/guide/mac-help/disks-you-can-use-with-time-machi ne-mh15139 )

Also the last section of Prepare for APFS in macOS High Sierra - Apple Support

I now have it working so that my old Mac running El Capitan can do a Time Machine backup to my new iMac running High Sierra. I succeeded by creating a folder "Network Backups" on my Time Machine external drive and sharing that instead of the whole Time Machine drive. Make sure you enable AFP in addition to SMB when you enable File Sharing. Note the AFP file sharing URL which will be something like "afp://<IP address>". Add the folder you want to use for network backups and enable it for Time Machine backups.

On the older Mac running El Capitan use Connect to Server on the Finder Go menu and enter afp://<IP address> as you saw on the High Sierra Mac. A dialog will appear with the available drives on the High Sierra machine and the folder you set up for backups. Connect to the shared folder and you will see it mounted on your desktop. Go into Time Machine Preferences and you should now see it as a choice when you click on Select Disk.... You don't have to leave the network folder mounted as Time Machine will remember it and mount it as needed.

Could this gave something to do with removal of the AFP sharing protocol in high sierra?

I also have a Mac running El Capitan that can't see a Time Machine drive shared by High Sierra. Sharing it using AFP instead of SMB did not help.

This doesn't really answer the OP's question.

See robinhood4's answer below.

Thanks - this worked for me backing up my wife's MacBook Air running Yosemite to my desktop running High Sierra.