This bug report is similar to bug #228715, but slightly different.
If I understood it well, old ports are allowed to keep "Created by:..." lines but new ports are not allowed to include one. This is based on my experience.
It still happens that new ports creators try to insert "Created by: ..." lines (as a new FreeBSD contributor, I did it on my first new port less than 6 months ago): the handbook discourages it, but the ports tree includes many "bad" examples. Such examples create confusion and might make new contributors feel bad when seeing that they cannot sign their work while others did (I felt a bit bad at first, before understanding the policy).
Furthermore, if someone interprets this fact as "once signatures were allowed, now they are not any more", one could think that (s)he should remove existing lines from existing ports (like I recently did in bug #233662), which seems to be against the official policy (unless the creator has been asked).
Specifying the correct policy once and for all in the porter's handbook is the best way to avoid similar problem in the future: if what I think to be the official policy is wrong, feel free to correct it.
(My personal opinion is that we should remove all signatures, both old and new: ports change over time and creators ends up with a signature that stays on a Makefile that can be very different to what they wrote when they left maintainership. Credit for creation can still be recognized through other means like Bugzilla or SVN. This policy also has the advantage to be fair between old and new contributors.)
Created attachment 199719 [details]
Makefile signatures patch
This is the patch I propose for the handbook.