The type of keys in maps is always string, even for numbers with units.

Example:

$map: (
    600px: 20px
);
@each $key, $value in $map {
    @error 'value #{$key} type #{type-of($key)} -- value #{$value} type #{type-of($value)}';
}

Compiled with scssphp:

value 600px type string -- value 20px type number

Compiled with compass:

value 600px type number -- value 20px type number

The spec says:

Numbers, which may or may not have units, like 12 or 100px.

https://sass-lang.com/documentation/values