Starting July 30th, you will no longer be able to log in to a Curse account that was not merged with a Twitch account. If you have not yet merged your Curse account with a Twitch account, please do so here! Otherwise, your account and its content will be inaccessible.
Right now, only Nintendo's own games can use it. Any other game will have to have an update to add Video Capturing. I don't know why games have to "support it", but it could be added in the next update or the Better Together version of Minecraft.
In fact, the games that support it right now are: Splatoon 2, Mario Kart 8 DX (v1.3.0), Zelda: BOTW, ARMS, and Super Mario Odyssey.