are these lumped together under semantic change?

Well, they are all kinds of semantic change. I've just don't remember the terms "semantic dirft" amd "semantic shift" from my readings. As to sanction and decimate, the OED has citations all from within 100 years of one another (16th and 17th centuries) for both. Decimate seems to have been learnedly borrowed into English directly from Latin, but sanction entered from French. Not Norman, but as a new legal term from Parisian French. One of the problems with the study of semantic change, is that semantics doesn't have much of a theoretical framework, having been ignored in the structuralist and generative phases of linguistics rather intensely, to work with, so the terminology may be laxer than in semantics or morpho-phonological studies.

Ceci n'est pas un seing.