Just in case this isn’t completely obvious:
CREATE TABLE some_collection ( id VARCHAR NOT NULL, data BLOB NOT NULL );
There you go - you can serialize any key-value pair you like, without a schema. Heck, the values don’t even have to be of the same type.
You may or may not want to do this, but you can. My point is, while there are many good things to say about NoSQL databases, schemalessness isn’t the interesting one. Enforcing a schema is a feature. Not enforcing one is not.