ZBmath (formerly Zentralblatt für Mathematik) will become "Open access" in 2021 (see for instance at EMS site and at FIZ Karlsruhe site). It used to be under paywall, although some partial access was allowed.

My question (which was suggested in the comments of this answer to Is a free alternative to MathSciNet possible?):

What are guarantees (technical and/or legal) that the reviews will remain in unrestricted universal access?

(Concretely, I'd be reluctant to write reviews if ZBmath has the possibility to suddenly turn back to partial paywall in 2030 for whatever reasons, especially if this paywall affects searches for reviews made during the so-called open-access period.)