1. Install the Userscripts extension for Safari, open the app and go through the setup as instructed by the app. Don't forget to activate the extension for Safari.
2. After fully setting up the Userscripts extension, go to the TwitchAdSolutions git repo and click on userscript next to video-swap-new. You can play around with vaft as well, but video-swap-new works way better on iOS in my experience. This script replaces ads with a lower resolution stream.
3. Install the script by opening the Userscripts extension in Safari while the script you opened in step 2 is the active tab and clicking the Install button.
4. Go to twitch.tv and enjoy your ad-free experience.

Optional:

• When you have the Twitch App installed, Safari displays the annoying "open in App" Bar on top of the website. This also leaks into Fullscreen Mode. To get rid of it, uninstall the Twitch App and optionally install another extension to remove those Bars altogether, like Unsmartifier.
• Want 7TV/BTTV/FFZ features like Emotes or a customizable Twitch Experience? Install the FFZ userscript, reload twitch.tv and configure your experience and/or install Add-ons like 7TV Emotes by clicking the new icon top right.