https://www.getunleash.io/

It would probably take some effort to hook it up to whatever billing platform you use, but it would make it easy to turn certain features on/off based on billing. Instead of checking if a customer has a specific plan, you instead have Unleash handle the features that are on/off and then check for those flags in the code.

I run a SaaS which has a feature system. Every plan has a set of features. There are additionally feature overrides for users. Checking if a feature is available to a user is a set membership query.

With a couple more lines of code I also generate my entire pricing (plan comparison) page, so that each plan shows all available features.

Not sure where a library would provide any value, there is barely any code for this (I mean, it's Clojure, so it's very succinct and expressive, but still).