I believe that it's not a protected term or a trademark or anything, but rather it's the case of misleading marketing. Open source is widely understood to be a specific thing, which Liquibase explicitly isn't.
Although, on the other hand, "Two years after release, the license for each applicable version of Liquibase Community code reverts to Apache 2.0". So, it's like... eventually open source. Which is still misleading, as it doesn't apply to the current versions.
> Open source is widely understood to be a specific thing
I think everyone thinks the way they understand open source is the way everyone understands open source. And yet every time an open source project, by any definition, changes their license, people debate what open source really means.
Unlike a term born with a specific definition, like "FOSS", open source doesn't really have a definition. The OSI has a definition that seems to be most popular, but that's not the only understanding of the term that is doing the rounds.
For plenty of people, open source means "source-available software". For others, it's software licensed under a subset of specific licenses (which licenses is also a subject for debate). And for some it means "software developed in a specific way that involves the community", like many Linux adjacent projects are, disqualifying corporate projects licensed under those specific licenses because they can do a Liquibase any time they want and there's very little chance a community large enough to maintain and develop the existing code will stand up when they do.
Liquibase now falls under one of the three definitions I've heard people agree about rather than two.
I guess you're right, in the way that we can either describe and prescribe language, and yes, open source is not defined in exact terms. I still do think that attaching the label to a product, and disregarding the decades long collaborative nature of the development is utter nonsense. In my opinion, the onus is on these people to get closer to what open source really is, not because I don't want to update the term, but because I want the movement to succeed, and not to be watered down.
"Open source" was a widely used term in several industries before it was ever used for software. The original meaning was simply "publicly available". It was OSI that attempted to co-opt the term to mean something more specific.
IT is like that in every aspect of its naming, I think. It's a separate domain. We got architects, rockstars, viruses, windows, icons, etc and they resemble the original thing only to a degree.
Although, on the other hand, "Two years after release, the license for each applicable version of Liquibase Community code reverts to Apache 2.0". So, it's like... eventually open source. Which is still misleading, as it doesn't apply to the current versions.