An extension system is currently under development, and you can view the latest work here. There is no official support for extensions in the base JSON:API specification.
JSON:API previously offered experimental support for a different extension negotiation system than the one now being discussed, and it provided a number of extensions for use with that old negotiation system. However, this system was always experimental and has now been deprecated.
New APIs should not use the old system or any extensions designed for it. APIs that already use these old extensions should direct clients to an earlier version of this page as documentation.