Aftermarket sells them cause folks think they are needed or are a great idea........WRONG!
Slideouts are self supporting when fully extended.
Now some are not designed to be walked on when retracted due to floor lifting.
But extended they support themselves as designed.
Supports can cause upward pressure on slideouts which at minimum could cause them to not properly seal, could cause misalignment issues and in extreme case cause damage.
So 'No' to slideout supports