Relational databases (sans JSON columns) are limited in the same way that seatbelts limit your ability to be transfenestrated during a crash.
Having a rigid and well-designed schema is a mechanism to keep you or your team from doing stupid things.
I think most would infer that, but in any case, it most definitely implies a non-rigid schema.
Cassandra et al. IMO only fall under the NoSQL banner by retconning the meaning to be “Not Only SQL.” Columnar DBs are a fine idea for certain uses.
Document DBs and/or chucking everything into a JSON column, though… those can die in a fire.
the only thing nosql means is that there are no relations. mongodb 8 and newer for example support schemas and validations, cascading checks, etc. dynamodb, more relevantly does also support a schema, and in fact you can't even create a table without defining one.
nosql does not imply no schema