The fully gold plated ADSB-out is being forceably mandated in the US effective January 1, 2020, but GA uptake is low and the capacity of avionics shops to get this done for the entirety of the GA fleet is doubtful
The entirely of the US GA fleet won't be affected because the planned 2020 implementation is only for those areas of the US that currently require transponder Mode C: in other words Class C and Class B airspace, including the associated Mode C veils. You do not need a transponder to use US Class D or lower airspace, for instance most airport control zones, which are Class D, or any Class E.
After the debacle with Mode S, it is reasonable that people are waiting to do anything about ADS-B Out. Mode S was heavily debated and never meaningfully implemented in the US below 18,000 feet. Few US GA aircraft have it, or ever will. ADS-B out looks a lot like 'Mode S revisited' to a lot of owners in the US. I'll be waiting until after ADS-B Out is actually legally necessary before I do anything - one of my planes is waivered I'll just fly that one for a while if need be.