When will Supports and ADCs understand this?

That doesn't really address the reason that AD carries have always been the one to take heal though. ADCs have taken heal because A. it means that support can take exhaust or ignite, which can be more useful when roaming than heal would be and B. it guarantees that the ADC will get healed in a teamfight, even if other people have less health. That's not guaranteed if the support takes heal.

Also Taking heal on Soraka ensures she always gets the health from it to use for her W

When will you understand that summoner spell choice is impacted by more than a single factor?

There's more to who gets which spell than Grievous Wounds. It's important to ensure that ADCs get the biggest heal possible, absolutely, but there are big trade-offs at play, primarily tied to when the support is not spending the entire game at the ADC's side.

ADCs take heal rather than Exhaust because Exhaust is simply, in comparison, lacking as a "keep me alive" spell. Needing to get in range of an enemy to use it is very iffy when a large number of the champions that are most adept at killing ADCs like to do that before ever getting in Exhaust range or, by the time they get in Exhaust range, cannot be peeled before killing the ADC.

On the other hand, taking Heal over Exhaust/Ignite denies supports the opportunity to make summoner-enabled offensive plays around the map. Heal is almost pointless when trying to get an enemy killed, which cannot be said for either Exhaust or Ignite.

Yes it's nice for the ADC to be a bit more resistant to GW but it's not worth it for making the ADC less capable of self-peel when alone and reducing the support's ability to make plays elsewhere. You are gimping yourself in so many ways just so you can get a bit more value out of Heal a couple of times per game. It's nowhere near worth it.

Personally would almost like to see tp on adc if they don't go top since herald is so strong.

No one takes ignite bottom.

lately ive been taking heal as support even if i have to be duo heal i find it better to help other team mates

really depends on matchups...sometimes you want the exhaust as an adc to shut down assassins jumping on you while your support is somplace else, sometimes you want the heal to have control over the ms-buff to escape/dodge/chase.