Puppet Server 7 is compatible with Puppet agents version 4 and above.
In Platform 7, Puppet Server removed support for the old Puppet 3 HTTP APIs. This means that Puppet 3 agents cannot be used with primary servers running Puppet Server 7.
It also removed the ability to opt in to using the legacy auth.conf file.
All endpoint access must now be controlled with the HOCON auth.conf file.