It's certainly less common now, but prior to 2015 it was common for "legacy" vendors (y'know... the paranoid and posessive types, like ProgressDB...) - or just those embarassed about the state of their products compared to the rest of the industry: they'll hide their KB articles, bug-reports and workarounds, and more besides) behind a registration-wall, or even worse: only grant you access if you have a current support contract with them.
At least their SQL documentation is public - I remember (at least a decade ago) that only Postgres, MySQL, and MSSQL made their documentation public, while info on IBM DB2 and Oracle was scarce.
