Why do I keep hearing this? It's not true. She doesn't have any buffs for allies and her one debuff is a slight slow aoe. Ashe is more of a "support adc" with her E. Sivir can tank a spell or effect. Vayne can cc people. Where in the world did this idea of support adc even come from?
Sivir is also a 'support adc' in the sense that she can buff herself and her allies movement speed through the use of her ultimate.
In the same sense Pyke is classified as a support, MF delivers a lot of supplementary damage to help her team net kills. Kinda like how Annie, Brand, and Zyra are except without the Hard CC. It can also vary in what support means to your teams composition. It's common for pro teams to build tanky for top lane and junglers with most of the damage responsibility being delagated to mid and bot lane. And when or if ever the tanking role has to bleed over to mid lane (much like we've had to see some teams pick Galio mid, then support might need to pick up a more offensive role to supplement the team that's lacking in damage but already excelling in hard CC.
Honestly anything can be a support role if you can make it work, as long as your playing to your teams strengths and helping your team generate gold leads, you're doing a good job of being a support. Even if you have to carry you ultimately translate income into more damage for your team to where you can carry or spread the wealth for the teammates who are behind.
The reason you keep hearing that is because a few years ago she was used in professional play by ROX Tigers to counter a very specific problem. Today people in low ELO still try to stuff her into the support category even though she doesn't quite fit.
At the time Zyra was extremely oppressive. No traditional support could contend with how easily she could zone with her plants. MF scaled with ability power, and more importantly she could poke through Zyra's plants while procing Spellthief's Edge. She was also picked with Ashe, who brought utility to the lane and they would combine ultimates for team fights.
MF support isn't something that can work against every lane. For example playing MF support against enchanters will result in an easy win for the enemy bot.
Ashe was their last true attempt at an marksman support, but that also never quite worked outside of specific situations.
People in low ELO don't understand that off-meta has to have a purpose either to counter something or because champions have insane synergy with one another (duo mages bot is now a thing, Garen and Yuumi were a thing for a bit, etc.). Meta stands for most effective tactics available, and if you play something subpar into that, then you're likely to lose. This is a strategy game.
Senna is an upcoming champion release that seeks to finally create a true "support ADC," which again has sparked popularity in previous examples like MF and Ashe.
MF's abilities are really great for safely lane bullying without the adc needing to help at all. She's most likely the strongest "caster" support you can bring into lane. On her own Make it Rain + Comet will cause bot lanes without heals to base numerous times, Double UP and Bullet time are also great for this to make the enemy unable to CS. She can lock out lane by herself, force multiple zones and your adc can safely farm and secure kills. It should be a guaranteed lane win in the early game unless the enemy has healing. Bullet time is always good also even on a support budget. The biggest thing is that all her abilities can be easily hit from an entirely safe range and are all "zoning" abilities and she's a a big threat to opponents under tower.
The issue with playing her support is that it tilts people so hard that they troll and throw the game if you pick it. I think she's fine at least in Gold or below unless the enemy team has a lot of mobility on assassins, and your ADC needs stronger peel or a long stun to survive. like vs a Talon/ Kat/Blue Kayn etc. Make it rain and bullet time can be enough to keep the ADC safe in a lot of cases.
You have players also that feel like playing a pure "support" support is risky because you have to depend entirely on your team being able to perform to some extent so a lot of support mains prefer playing things that can convert if needed into a damage carry, like Lux, Brand, Annie etc. I don't think it's wrong to look at it like that because supporty supports like Braum, Leona, Rakan are going to find themselves on a lot of teams where no one can damage or carry and then you would just auto lose. If you did pick a supporty sup it's not like you suddenly feel like the game is much easier to win you just feel far more dependent on the performance of your team mates.
