Can no longer delete photos from within an album on Photos for Mac

Select the images you want to delete and use the key combination of ⌘+Delete. That will move the files to the Recently Deleted album where you can remove immediately or let them stay for 30 days before being deleted automatically.

OR if the hold down the ⌘ key the Remove from Album changes to Delete.

The command-delete ⌘⌫ key combination, pointed out by Old Toad, does still delete the selected photos anywhere. It is very convenient and I am using it a lot, also holding down the command key, to change the contextual menu. But it has a drawback (or advantage, depending on how you view it). When you delete with ⌘⌫ from an album or smart album, the photos will be deleted without any prompt "delete from all devices ". You have to check the number of selected photos in the upper right corner of the Photos window, before you use this key combination. Photos 5.0 on Catalina macOS 10.15.1 tends to keep photos selected, when switching between views, and there may be many more photos selected, then you are aware of. So check the number of selected photos in the toolbar, before you use ⌘⌫ .

For example: In the album are two photos selected, but the status message is showing four selected photos:

I added this as a new user tip: How to delete photos while viewing an Album in Photos 5.0 on Catalina - Apple Community

Photos are never stored in the albums you create. You seem to fear duplication. That does not happen in any of Apple's photos apps.

They remain in all photos. Albums are merely "tags" given to photos for them to appear elsewhere. They are never moved to that other place. When you create a shared album, copies are made and moved to the Apple provided storage space where shared albums are hosted.

'This is why "remove" from album is available but not "delete"