Okay so, couple things here.
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This seems to apply pretty much 1 to 1 with regards to Windspeaker's Blessing as well, and yet you don't want that removed? In fact, I feel like most of the community wouldn't want WSB removed, as although healing can be annoying having something for a healer to take is important. You can make the argument it's different because tank supports can take one of the other keystones in the tree, but it's not like Janna could never proc thunderlord's. Protecting an ally is a seperate enough playstyle to warrant a keystone.
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There are only six champs using it. But I believe that's not the mastery's fault, and instead a twofold problem.
Firstly, support itemization is bottlenecked at locket currently (which is getting fixed soon), so if you want to be buying something else you're simply not a good choice atm.
Secondly, supports tend to be more 'tier-locked' than other roles. What I mean by that is, playing a lower tier support is much less uncommon than other roles. Mordekaiser is a horrible champion speaking strictly powerlevel wise, but you can one trick him and be fine. Maybe some games you do bad and people get angry at you for picking him, but if you do well people won't say shit. With supports, you have to lane with someone else, so if I want to play Rammus support, in addition to any other flak I might get from picking a bad champion (which would also apply to someone picking a bad champ in any other role), I have to deal with the adc being both much more likely to be upset at me, and more upset at me than my allies would be if I were in a solo lane or jungle, because he is much more invested in my pick, he's laning with it after all.
And then, after champ select, if I'm doing neutrally, not making insane plays but also not dying unnecessarily, etc, I'm still more likely to be harassed because the adc is constantly reminded during lane phase that, for example, "If we had any support with mobility we'd be able to dive here, but since this guy picked Rammus we can't." Regardless of the potential positives of such a pick, the negatives are so obvious and in-your-face (the adcs) that as a Support player, someone who wants to help my allies, I'm heavily encouraged to just pick a high tier support.