PBS just launched a new free ad-supported channel known as PBS Retro. It will be playing classics like Reading Rainbow and Mister Rogers' Neighborhood.
The article currently cites it on Roku Channels, but I was able to find it in Channels from my Plex source.
Source:
https://www.engadget.com/pbs-retro-is-a-new-fast-channel-playing-just-the-classics-173125500.html