Armin from Sentry here: obviously biased but the term I like the most to describe BSL as used by Sentry is: eventually Open Source.
The BSL upgrades automatically to Apache 2, so the only thing that it does is establishing a time window where there are some restrictions on the code.
A theoretical version is one where you just only release code with a large delay under the Apache 2 license and keep the rest closed source. That's also still technically open source but I would argue that's not a particular good version.
