They did, and you can still find those yellow and green flags in some match officials' changing rooms.As I recall the Isthmian league used to use green and yellow flags rather than red and yellow. The clubs had to provide them rather than the match officials. (At the time no one used buzzers)
I think they stopped this about 2002 when the contributory league boundaries were redrawn and officials were appointed by the FA rather than each league.
They did, and you can still find those yellow and green flags in some match officials' changing rooms.