The Jedi becoming completely forgotten after only 20 years between the PT and OT is weird, but you can sorta handwave it away with the idea that the Jedi could have been discredited and smeared by the rather biased dictator running the galaxy during that time turning them into traitors and charlatans.
I believe they've also established that nobody in the galaxy outside of Yoda, Obi-Wan, Vader, and maybe high ranking imperials even knew that Palpatine was a Sith or even able to use the Force.
By the time of TFA, it's been 50 years since the Jedi Order was a thing, and unless you were living on Coruscant or a contested world in the Clone Wars where you happened to see a Jedi in combat, it would have been like 0.0000001% of people in the galaxy who ever saw a Jedi in person.