Relevanssi doesn’t know how some random membership plugin protects posts. It can handle s2member out of the box, and Groups from the latest version on, but it can also be taught how to support other plugins.
Simple Membership is easy to support and will be included in the next version. If you want support for Simple Membership now, you can use this: https://gist.github.com/msaari/daa1c9bdceacdfe61e130bbbf42409c2
But that doesn’t cover Partial Protection. Are you sure the Partial Protection doesn’t already work? Since it’s done with a shortcode, it’s possible that it would just work with Relevanssi, as Relevanssi runs the shortcodes before creating the content to excerpt.
If you’re logged in, you should see the protected content in excerpts, but if you’re logged out, you shouldn’t. Please double check. If it really doesn’t work and you can see protected content in excerpts even when you’re logged out, then something can be done – the easiest solution is to simply remove all protected content to all users.
Do note that Relevanssi will index all partially protected content and will return posts if someone searches for the protected content, even if it can’t show it in excerpts. If that’s a problem, then it’s possible to make Relevanssi not index the partially protected content, but in that case nobody can find the posts with that content, even those with permissions to see it.