Why cant I use [^index] to index from the end of an array, even though I'm in c# 8.0?

On the microsoft c# docs website, it says you can use [^index] to index from the end of an array in c# 8.0 and later, well, I’m in c# 8.0, but it doesn’t work. I’m wondering if it’s possible to make it work by updating Unity, or if there’s some other solution. I’d also just like to know why this doesn’t work, even if there’s no solution.

C# 8.0 seems to be supported with Unity 2020.2 (currently in beta)

Unity 2018.3 → 2020.1 support # 7.3

I’m on 2022.3.20f and it’s still not supported. Can’t do myList[^1] or any kind of negative carrot indexing. I’m assuming this is just my ignorance between the differences of C# Language Version, C# Compiler, and the .NET Runtime.