Supports are the backbone of every meta. Their cooldowns and ults enable the rest of their team to get value. What character you pick and how you play that character with depend entirely on what your team composition is. Even just picking the incorrect character within some team composition can be the sole reason that you lose games. As a support player, you need to flexible. If you wanna be a one-trick, go play dps.
I think to start, it's important to make the distinction between main support and main healer. A main healer is character that outputs a lot of healing. A main support is support that seldom changes because it makes a meta work. Examples of this are Lucio in brawl and Zen in poke. There isn't a list of main supports because there is overlap between main and off support. The reason a support player needs to be flexible is if a meta shifts, you need to be able to play either the main or off support to make a composition work as you supply the utility to enable the tanks. Brawl doesn't work that well without Lucio's speed and Poke isn't great without Zen's discord.
The off support is the character that is more flexible in swapping in a meta. Think Ana and Bap in brawl or Bap and Brig in poke. This role is often reffered to as Flex support. The term "off support" is a bit outdated, but it's an easier idea to grasp when you're just starting. In fact, the whole idea of Main and Off support is getting a bit greyer as time goes on. Over all the past metas and because the support hero pool is so small, many high ranking players can proficiently play nearly every character. That is where the term "Flex Support" comes from. For now, it's important to start small and specialize in just 2-3 characters.