Just finished a call with Apple Support. Verified that there is no way to delete the persistent legacy "My Places" with the usual right-click & delete. Then verified that iCloud Sync was enabled for Maps on all my devices (iPhone 13 Max Pro, iPad Mini 6 and MacBook Pro). Verified that all devices were running the latest OS. Tried a number of minor things that made no difference.
I had already cleared Significant Locations history on my iPhone and iPad, but had not done that on my MacBook Pro. When I did that on my MacBook followed with shutting down and restarting Maps, after approximately 2-3 minutes the single legacy location under "My Places" disappeared, and so did the "My Places" guide!
The Apple technician seemed just as surprised as I was when we noticed "My Places" was gone. :-)
Recap: The trick seems to be to clear history on "Significant Locations" on ALL your devices.
On iOS devices, go to Settings / Privacy / Location Services / System Services / Significant Locations and touch "Clear History".
On macOS, go to Settings / Privacy & Security / Privacy tab / System Services (Details...) / Significant Locations (Details...) and click the "..." inside the small circle in the lower right corner of the left side of the "Recent Records" display (a map will appear on the right side of that display). Click "Clear History" to wipe out all Significant Locations from the Mac:
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