RDAP is a protocol; everyone has "access to it". ICANN provides a web client.
ICANN is trying to make all registrars support it. The article suggests this effort is now pretty well along, though the first two domains I tried to look up failed RDAP lookups. (The third succeeded.)
Apparently the RDAP agreement restricts access to some PII and allows for further restrictions, but I couldn't be bothered to read the details. The RDAP FAQ does suggest that there may be more information available to "authenticated users" than to anonymous ones. I suspect we'll have to see how it all unfolds.