It's unnecessarily complicating things to require case sensitivity here. Words don't typically completely change their meaning just because of capitalization. And suppose I write that in a Slack channel where no one uses caps at all? Do I have to use caps anyone to make sure I'm not confusing everyone? How do I pronounce it correctly if I'm giving a speech such that listeners know which one I mean? What if the closed captioner writes the case wrong?

Nah. "Open Source" = "open source", because any other interpretation goes against the norms of written and spoken English, and because it'd be an absolute freaking pain in the neck to create that brand new distinction that's not an issue today.

The term should've really been "Libre source" and that would've been very much in line with the idea behind it. Alas, that boat has sailed.

Sure, if you invent a time machine and rewrite how things actually evolved.

Early on, you mostly had only two kinds of code: Proprietary software whose source code is closely guarded as a trade secret, contrasted with open software where the source code is quite deliberately shared with the world as widely as possible. The former was code owned by companies, the latter was generally academics and hobbyists.

It's only somewhat recently that there has been a fairly large gray area between those two, mostly from companies who want to capitalize on the warm fuzzy feels of Open Source in their marketing material while building a moat that doesn't allow others to do much without the missing proprietary bit, or because the license doesn't allow redistribution, to pick to random examples.