Plex doesn’t sell contact data of its user base to anyone.
I took a look at your account. This is very old. I assume you were using this email address for a very long time already.
There was a hack of the plex forum system back in 2015.
See Getting spam emails to my plex email address
The data which were siphoned off contained email addresses. These data are now floating around the interwebs and can be used by anyone. There is nothing Plex can do to prevent it.
It looks to me that somehow Netflix saw fit to use email addresses which were gathered in such a disreputable way.
IMHO a very questionable action by them. It maybe worth asking them directly how they usually acquire marketing email adresses.
