Support is more of a generalization than hardset class; it's probably one of the last positions besides jungle to have several potential picks. For example (assuming your team isn't hostile and has synergy), you could run atrox as a support by setting up kills for your team with his crowd control; however if your team wants more sustain and poke then you could run nidalee as a support.
Problem is that some champions have more tools, or more useful tools that make them better supports. Alistar for example can stop enemies in their tracks like atrox, but can also heal his teammates; janna like nidalee can poke and defend her ally, yet janna's poke is also a crowd control. Another example would be blitzcrank, originally a top lane bruiser back when he came out; but as ranged physical carries got more mobility, he ended up relegated to support, since he excelled at holding them still.
On the other hand, some supports aren't particularly good paired with some carries; such as braum & graves, sure they're both close range, but because graves has to reload, it'd be rough for them to consistently use braum's stun. Whereas lucian & braum are complimentary since lucian can get an extra auto-attack for free, making braum's stun happen more often.
Again if you have friends willing to be flexible, you'd be surprised what can work; it's not just what support is good, but also which support will empower your carry without much resources (as supports are often the most behind). The most amusing example I had seen was yasou bottom with ahri support; sure she had very little damage, but since she rarely used anything but her charm, she didn't run out of mana.
While some champions are clearly meant to be supports and nothing else, like soraka & sona, there's still plenty of room for other champions to be played. Yes most of it'll be opinions, but firsthand experience can be the best teacher for you; seeing which combinations can work together, as well as which champions are flatout ineffective in other roles.
