Shadowing sounds not good, it's a gateway to unneeded bugs and as a result it will be harder to debug.

As to javascript developers the current let is not the same let they are used to, so it's safe and easy to get used not to use this keyword at all or use when appropriate.

Regarding rec and and we can still use let, but in other cases I would propose make it optional.

Shadowing also prevents bugs by removing obsolete variables from scope, and avoids having to make up new names for variables with identical meaning. In a functional language where mutability is discouraged, that's quite convenient, and natural.

In a functional language where immutability is discouraged, that's quite convenient, and natural.

But Immutability is the basis of functional programming, it should not be discouraged.
Overriding variables in a function seems more like a terrible design/habbit from imperative languages thus a way to produce poor code.

Sorry, I mean mutability.

With a mutable variable you can update it, e.g. a = a + 1.

With an immutable variable and no shadowing you either need to create a new new name, e.g. let a''' = a'' + 1 (good luck getting the number of apostrophes right), or you skip the variable entirely and instead write large unreadable expressions. You're of course free to think this is good style, but you're not making a very good argument for it.

With shadowing you keep the name short and meaningful and remove the obsolete value from scope. No unwieldy variable names and no bugs from accidentally using obsolete variables.

It might be nice to have shadowing disabled by default and have to opt in to it per definition though. But I haven't ever had a bug caused by shadowing, so I wouldn't personally give it much priority.