Seeing your post my first thought was that multi-image albums always worked on old.Reddit. But actually I was thinking of a Reddit Enhancement Suite feature. Which is another open-source toolkit that was provided by the community to Reddit.
Happy to see that the Open Source came up with something top-to-bottom like Lemmy and the Fediverse!
I'm sympathetic to your idea of calling it OSS or Open Software. But Richard Stallman and people who agree with his arguments really stress the "freedom" of what they call free software. They lost that battle ages ago, but they aren't going to give it up since it's more than just pedantry, it's a value statement.