According to the channel lineup listing Smithsonian is encluded in the Economy (100+ channels), Preferred (220+ channels), Preferred Plus (230+ channels) and Premier (260+ channels) packages. It is not in the Starter (140+ channels) package.
This is the only channel that has a gap like this. Every other one if it is in a particular package it is also in all the other packages that include more total channels.
It's also listed that way in the new printed guide for my area, effective date May 2018, that I picked up today when I was at my local Comcast office (Kitsap County, Washington) making some changes to my service. The person helping me there thought this was really odd, and spent several minutes checking things out on her systems, but could not find any indication that it is just a mistake in the guides.
Actually, there are several channels that are in Economy, but not Starter. I think it's because Economy doesn't have sports channels, so they filled in with several other channels.
The lineup may vary from area to area, but I find these in Economy, but not Starter:
Aspire, BBC World News, Smithsonian