Every campaign has requirements per platform set by the publisher. Those requirements determine if you can request a title as well as what platforms you can deliver on. If you go to the campaign page for any of your accepted offers, it'll show your eligible platforms.
For the ones you won't be able to deliver on due to Mixer shutting down, things are a little tricky. In that scenario, we recommend you contact the dev afterwards with a link to your content for their game on YouTube or Twitch. Please do CC us (firstname.lastname@example.org) on those emails as well, as we'll be happy to reset your quota just as we normally would upon seeing that content come into the system.