What happens to my photos when I remove a device from iCloud and Find My?

If you remove your iPhone from syncing with iCloud the photos will remain on the device.

Set up and use iCloud Photos - https://support.apple.com/HT204264 - FAQ

Remove a device from Find My iPhone on iCloud.com - https://support.apple.com/guide/icloud/remove-a-device-mmfc0eeddd/icloud

"When you remove a device, it’s removed from your Find My iPhone Devices list, and if your device has iOS 7 or later, Activation Lock is also turned off."

Before you can remove the phone from your iCloud and Find My you will need to erase it, so yes it will erase all photos on the device.

If the device is at the Activation Lock Screen, then any data including photos on the device is already gone.

Removing the device from Find My will remove Activation Lock. however, if you can do that, you should be able to just enter the password for the Apple ID on the device itself to remove Activation Lock. You won't be able to remove it from Find My if you don't know the password for the Apple ID its expecting in the Activation Lock Screen.

It isn't mandatory to erase a phone before removing Find My and iCloud. Usually one does do it but it isn't required. In fact I inherited a phone where the spouse asked me not to erase photos until we were sure she had them, so I signed out of Find My and iCloud so I could set up the phone for my use but the photos stayed on it until I was told it was okay to erase the device entirely.

True. My mistake.

Still if it's already at the Activation Lock Screen, the point is moot.