Can a Kotlin 1.4.30 library with experimental language version 1.5 be used with Kotlin 1.5 once released? 🤔

No. It’s similar to using a 1.5-M* preview. See pre-release notes here https://blog.jetbrains.com/kotlin/2021/02/new-language-features-preview-in-kotlin-1-4-30/#how-to-try

Ah I’ve missed that info. Thanks 🙏