Removing Floors doesn't work how it used to

Basically your second and first floor rooms need to match. So, you have to remove both the floor of the second floor and the ceiling of the first floor. If they don't match, it won't be removable unless you make an even smaller room within that where you want there to still be a floor/ceiling.

First make your rooms, one on top of the other:
https://i.imgur.com/yX3eLoZ.jpg
https://i.imgur.com/IY65ktI.png

Then remove the floor of the top story:
https://i.imgur.com/CSpkl5d.jpg

Then you need to click the yellow portion to remove the ceiling of the bottom story while on the top story after the floor has been removed (you may have to "remove the ceiling" then "build" it, then remove it again for it to actually go away):
https://i.imgur.com/2RQsmHW.jpg

You're left with an open floor where you can add rooms and doors and such as you wish:
https://i.imgur.com/gYL3Pwa.jpg
https://i.imgur.com/0IcIg90.jpg
https://i.imgur.com/vaJ25o0.jpg

If you want a grand entrance, make a room in the hallway's shape around the edges and build the ceiling:
https://i.imgur.com/L4yRQtT.jpg

Then remove the walls on the lower floor's hall:
https://i.imgur.com/Z6i97c8.jpg

Then you can add doors, furniture and design to your heart's content!:
https://i.imgur.com/Whs3Hj3.jpg

Did you try the sledgehammer?