I see this. Is SSLKEYLOGFILE defined by the specific SSL package used by curl?
$ SSLKEYLOGFILE=sslkey.log curl -s https://httpbin.org > /dev/null
$ cat sslkey.log
CLIENT_RANDOM 73c0277fd99b097691bc1745f14376cf9cca3c75f357ce4d276de9402d17e1b3 1cccf53210ce60caf626c39e55bf988d2666146dd0597437ba3b3feb745f53360683e86e00f77c7f93068f63fc24f551
Or it is a standard of all implementations of SSL. For example, if I set SSLKEYLOGFILE as an environment variable, then call a Java program that uses SSL. Will the SSL key log be captured as well?