does the decoder su[pport cat021 message format

i beleveie that there are multiple formats for adsb , and among them cat -021 is hat standard one ,
does the decoder of open-sky support that in java

The java decoder (github.com/openskynetwork/java-adsb) is only meant for decoding Mode S and ADS-B transponder signals as defined by their standard documents ICAO's Annex 10 Volume 4 and RTCA's DO-260B. You can use this decoder if you operate your own receiver for example.

I think what you refer to are protocols used to distribute ADS-B information between different organizations or system components (ASTERIX CAT 21 for example). This is not supported by the decoder.