The AHBL was a list of known bad or misbehaving hosts on the Internet. It included hosts and networks that were known to originate harmful content, abuse, etc.
Public access to the DNSbl list was terminated in 2015.
The list continues on as a private invite only DNSbl and RHSbl (domain name based).
In simple terms, the private DNSbl and RHSbl is by invite only. The rules and conditions for using the private service are very different from the public service.
If the list opens up to more public access, information will be posted here on how to sign up.