This is indeed true.
I am one such collector.
Behold, a clipped film strip once used in the finale of the Original 'Journey Into Imagination' attraction.
Met someone who had managed to get one of the last sets of film reels used in the Original before it was closed in 1998.
One of the gems they had was a full reel of the animation used in the first scene you see on the ride before entering the turning staging area.
A example of the animated scene i speak of -
They also had the various reels used for the finale scene featuring Figment imagining himself in various dream roles, but had foolishly cut them all up into 'clips' in anticipation of selling them.
Said person must have assumed they could make more money selling clips instead of the full intact reels...which they surely must have learned otherwise when the one full reel they had was finally purchased !
Du'oh....!
Film reels and taped audio tracks would periodically be replaced, and the old used copies disposed of.
Many found their way home with fans who worked for the company, and many shared their treasures with others (particularly audio).
Back when Disney went all digital, the last of said film reels and audio tapes were ditched...and a few key people scooped them.
A few years back audio samples in particular started turning up in quantity and excellent quality, direct from original analog 'work copies' scooped.
One fan i know of made a stunning compilation of the all the musical audio tracks used in 'Horizons'..taken directly from ride-used tapes.
Another fan i am aware of scooped a load of EPCOT related background music and Pavilion BGM some years ago during the analog-to-digital source purge and put them out there for free for all fans to enjoy.
Today a lot of 'un-officially' released Attraction scores and general Parks music can be found if one knows where to look.