I find it interesting how there's so many ways now of describing mutable vs immutable.

Rust: let vs let mut Swift: let vs var Js: const vs let Silk: val vs var

I honestly get confused now. I started a new job working in go and I keep writing let everywhere

The only one of those, that does not indicate by name to me, that it is about mutability, is the on of Swift. I guess you could argue, that in math, when someone says "let x be 2", then x is constant. In the context of programming though, I am more familiar with the var and let in JS, which is not at all about mutability.

Anecdote: Kotlin uses val/var, but IntelliJ (the de-facto standard editor for Kotlin) will underline all mutable variables.

If you don't mutate them after initialisation.

Never had this problem with Scala...