(In reply to comment #0)

The group 'autoreview' is only created when $wgFlaggedRevsAutoconfirm is an
array; the default is "false". But since r100327 (fixing bug 31441), the
'autoreview' group is unconditionally added to $wgAddGroups['sysop'] and
$wgRemoveGroups['sysop'], leading to this nonexistent group being listed on
Special:ListGroupRights.

IMO, the best fix would be to make the $wgAddGroups['sysop'] and
$wgRemoveGroups['sysop'] entries conditional on $wgFlaggedRevsAutoconfirm.

That would then override any LocalSettings changes to the contrary, which is worse.

I might makes sense to always define the group, regardless of $wgFlaggedRevsAutoconfirm.

Fixed in r105872. The group now exists by default. If wikis don't want it to even be addable/removable, then the add/remove rights config can be set accordingly.