BSL is a source-available license that by default forbids production use. After a certain period after the date of any particular release, not to exceed four years, that release automatically converts to an open source license, typically the Apache license.
Projects can add additional license grants to the base BSL. EMQX, for example, adds a grant for commercial production use of single-node installations, as well as production use for non-commercial applications.
Edit: it's not a paywall. It's the standard BSL with a 4 year Apache revert. I personally have zero issue with this.