I've seen a set of SQL tables designed to mimic "flexible classes". There's a table for the "class", another table defining its "fields", and two other tables defining class instances and related field values (all as varchar).

Flexible, yes. You can store anything. The downside is, that I've also found "anything". Stuff attached to the wrong "class", wrong datatypes, missing "obligatory" fields, etc, etc.

It's a PITA to work with. If I could design it from scratch, it'd be a single table with JSON payload.