I'm creating an updated JSON Schema for the 1.0 spec, and re-read this section of the spec, which is similar / the same in v1.1 in relation to attributes...

Complex data structures involving JSON objects and arrays are allowed as attribute values. However, any object that constitutes or is contained in an attribute MUST NOT contain a relationships or links member, as those members are reserved by this specification for future use.

Bluntly, this is wrong. It's unreasonable to place constraints on the attributes of entities that you have no control over.

Say, for example, my system represents patient data, and those patients have relationships which I express in an object under a relationships key. According to the spec, that entity can't be legally represented in the JSON:API spec format.

reserved by this specification for future use

Making use of or applying semantics to specific attributes in data you have no control over, is asking for trouble.

Could someone please explain to me the motivation behind this specific section and language, please?