Is AMDAR a required system on aircraft?
AMDAR (Aircraft Meteorological DAta Relay) is a program established by the World Meteorological Organisation to gather weather data from around the world. It's run through a number of local and regional AMDAR organisations who negotiate with partner airlines arrangements to use existing aircraft sensors to gather and report weather data on each flight. 

The gathered data is relayed by satellite or radio to ground stations and then forwarded on for processing and distribution.

(I've paraphrased from this source)

So, there is no specific AMDAR equipment. The AMDAR system is implemented by arrangement with partner airlines, and may be installed on some or all of the airline's aircraft that already carry suitable sensors.

There is no mention of a requirement that an airline partner with a regional AMDAR organisation, and thus no requirement for their aircraft to carry the system.

It's not clear from the pages at the WMO what the nature of the collaboration between the local AMDAR organisation and the airline is, so there may be some payment to the airline, or there may be some benefit in kind: free or reduced cost access to meteorological reports, perhaps.