Whilst you cannot delete old devices manually, you can pause and unpause them. Old devices are supposed to get removed from the list after 60 days of being paused but I am aware there was an issue with that particular functionality, that if I recall correctly has since been fixed.
In any case, whether removed automatically or if you had the option to manually delete devices from the list, that device still knows your Wi-Fi network password and can re-join at any time if its in range. Deleting the device from the list does just that, deletes it from the list, its purely a cosmetic thing. To stop a device from re-joining your Wi-Fi network, you'd need to change the password.
