Discuss implications of API implementers making optional fields required

[dutabado]

We've discussed at detail scenarios where an API implementer might add additional required fields to their requests/responses that aren't part of the official spec, but we haven't discussed scenarios where an implementer makes optional fields defined by the spec required.

This originally came up under the context of compliance. However, since we've decided to defer having stances on (non-)compliance for V1, this is not an active issue.

We did, however, want to keep a record of this for future discussion so that we can decide what our stance is on implementers taking such action.