I'm pretty sure that it is legally enforceable in most jurisdictions, B2B contracts are almost always stronger/more binding than consumer ones (since that protections granted under consumer protection laws are gone, and there is a reasonable presumption that the parties have read and understand the contract). The blurrier part is for sole proprietorships, since it depends on whether their specific jurisdiction considers them as consumers under consumer protection laws (which uniformly weakens contracts to the extent that it does not comply with the guarantees for consumers).
(Note that in most jurisdictions, most "copyright licenses" are considered as contracts. For example, multiple cases in French courts have resolved that GPL2 is considered a contract.)
Also, the license text: https://mariadb.com/bsl11/
