Do the settings under cn=changelog5,cn=config (such as nsslapd
-changelogmaxage) have an effect on newer MMR environments?

Yes, they do.

Glad to know that.

The
documentation seems to indicate that the values under cn=changelog5 
are
only related to the older replication mechanisms.

Which documentation are you reffering?

https://access.redhat.com/documentation/en
-US/Red_Hat_Directory_Server/8.0/html/Configuration_and_Command_Referen
ce/Configuration_Command_File_Reference
-Retro_Changelog_Plug_in_Attributes
-nsslapd_changelogmaxage_Max_Changelog_Age.html

Because it's under the retro_changelog_plugin section, and the
discussions of retro changelog is described as:

"T he Retro Changelog plug-in configures Directory Server to maintain a
changelog that is compatible with
the changelog implemented in Directory Server 4.0, 4.1, and 4.1x.
Maintaining a retro changelog is
essential to maintain a changelog for directory clients that depend on
a Directory Server 4.x-style
changelog."

In the administration guide.

dn: cn=Retro Changelog Plugin,cn=plugins,cn=config
objectClass: top
objectClass: nsSlapdPlugin
objectClass: extensibleObject
cn: Retro Changelog Plugin
nsslapd-pluginPath: libretrocl-plugin
nsslapd-pluginInitfunc: retrocl_plugin_init
nsslapd-pluginType: object
nsslapd-pluginbetxn: on
nsslapd-pluginEnabled: off
nsslapd-pluginprecedence: 25
nsslapd-plugin-depends-on-type: database
nsslapd-plugin-depends-on-named: Class of Service