Senna is not a support, no matter what Riot says.

She is completely different from Pyke because she can ACTUALLY work as a solo laner due to the tools her kit gives.

Main reason is her Q. STUPIDLY large Range, equally stupid hitbox, and it heals herself no matter what, as long as it's used. The latter part in particular is why nobody will ever play senna as a support. It's too much free power. ALL supports have exactly one thing in common: Defensive choices are not global. Senna is quite literally the first support who can sustain both herself AND her laner without any conditioning whatsoever. Soraka for example has her own heal being over time, incredibly small, and can be accidentally passed on to someone else through her main healing Ability. Nami has to be in the heat of the battle for the bounce to hit her and by then it's much weaker than the main healing was. Taric has insanely high manacosts earlygame and can't affort to use it more than once every half minute or so earlygame and if he does spam it it becomes much weaker due to the stacking mechanic. Senna is the first support who's heal can cover the entire team that isn't somehow heavily gated behind skillful use.

Her Q heal should either be massively conditioned to fit the support role (Example, only heals her as well if she hits both an ally AND an enemy) or it should be removed entirely.

That and it shouldn't proc Onhit effects because of Klepto. So many people I have seen play Klepto Senna because it's even more abusable than Ezreal, and I speak in matter of facts if I say that NOBODY thought that could be intentionally possible. Well, how it stings to be wrong.

Her Q, as is, is an ability you'd actually expect on a modern Juggernaut. Juggernaut-Abilities on ranged Champions has never, in all the 10 years, ever worked. Every time self-sustain on an ADC became problematic it was immediately nerfed (See BORK Lucian Meta for examples) because of how much it upsets the meta and it's balance.

After Lucian, Draven and Vayne last season you'd seriously think Riot learned not to give ADC's too much free sustain. Like, our expectations were low but holy fuck.