What are the constraints for lacking encoder support in FFmpeg?

According to the documentation, there are several video and audio codecs with missing encoder support.

Is this typically because:

A) The encoder is copyright protected.

B) The encoder implementation is unknown (closed-source).

C) No one has so far needed or wanted to implement an FFmpeg version of the encoder.

From reading FFmpeg License and Legal Considerations it is not clear to me whether A is a concern or not for adding a new encoder to FFmpeg. If C is the case and A and B are not a problem, is my only option to get support for a missing encoder in FFmpeg to implement it myself or can I raise a suggestion to a developer somewhere for upcoming features?