Erase old iPhone with smashed screen

The only way to do it would be a remote erase via Find my in your iCloud account. If Find My is off, then you cannot do that obviously.

But the only other ways to erase it would be interactively in settings, or to plug it into a Mac or Windows PC and do it in iTunes or finder. But that requires you be able to unlock the device by entering the screen lock passcode, which you cannot now do with the damaged display.

So sorry, but there is no way to erase it now. The consolation is there is also no way to access the device data either.