Apparently, there used to be a 'remove' button for each device listed. Now, it appears that the only way to remove old phones I no longer have/use is to use 'Set up secure & erase', which supposedly sends a notification to the phone. That is useless for the phone I no longer have. The only other info I have found is that I need to completely reset my google account (new passwords, etc.), which would be a royal pain in the... There has to be a simpler way to remove unwanted devices, not to mention an easy way for both my husband and I to add each other's phone to the other's Find My Device, but that solution seems to be elusive, as well.
and from the three-dots-menu of your old device, choose exit
Then go here https://play.google.com/settings/
and uncheck "show in menu" for your old device