Even openapi extensions specific documentation enumerates a list of allowed places where extensions can be used, there are more resources that can support them, for example, Servers as its specific documentation shows.

Current kin-openapi version 0.39.0 supports extensions in the resources described in openapi extensions doc.

But would be useful to expand extensions to the rest of the resources that support them.

Note: This applies to v2.0 and v3.0 specs.