That is not true. Support don't' need gold, but XP is still very useful for pretty much every champion. It's true that many supports at high ELO sacrifice their XP for their teammates, but that's not because it's useless for them, it's because the vision they get allows their team to make plays/ or farm safely.
Many support scale really well with level (
for instance power spikes really hard at lvl 16), because they usually have utility in all their skills that scales up with level, while Damage dealers often only really care about the rank of their primary damage ability, and whatever skill they level up third is a one point wonder.
Support is chronically underplayed, and that results in much longer queue times for people who don't queue as fill or support. Helping reduce the XP deficit between support and the rest of the team would go a long way to making support a more fun role to play. It's really not fun when you die while warding because you are lvl 14 when the enemy Mid/Top is lvl 18 and you have so little HP that you are easily blown up.
Support already sacrifice their gold income, they don't need to sacrfice XP too.