[cthalupa]

It's source available. Which is not the same thing as open source.

[rowanseymour]

I don't think anyone is arguing that BSL is capital O Open Source, and even the BSL licence stressses that it is not an Open Source licence. But both "closed source" and "source available" feel like poor descriptors for a project that is developed in the open and takes PRs from outside contributors.

[iavael]

For this case there's already an old good word "proprietary" that covers everything that's not FOSS.