Twitch bought out Curseforge, and incorporated addon management into the Twitch Client in order to attract more people to it. That’s why the curseforge client doesn’t exist anymore.
The Twitch Client isn’t actually full of spam at all, at worst you might see one ad banner. The streaming features of the application can be easily ignored, as addons have their own dedicated tab in the app.
The Twitch client can do just addon management. It has all the features the curseforge application had, with the addition that it can update all addons at once instead of two at a time. Twitch has made the premium curseforge features freely available that way.