Correct. The last model that still had an SD card slot was the Ultra 4670. As @renojim states, they aren't required to work. They don't make the player any faster or more powerful. They simply add some additional storage if you have a large number of channels installed. If you install more channels than will fit in the existing storage, Roku simply unloads the least used channel and installs the new one. When you want to use that little used channel, it is re-loaded and the next least used channel is off-loaded.
In theory you might save 10-15 seconds while the channel is reloaded, but in real use since those channels are seldom used it's really not a big deal. Hence the removal of the slot from all devices.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.