When function declares Nothing as a return type, it means that it will throw an error

Not necessary. You can also declare function with

Nothing

return type and have an infinite loop inside.

When an expression declares Nothing as a return type, everything surely executed after that will be unreachable

I would not say everything here. If this expression is inside

try-catch

block, then everything after this block is reachable. Also

continue

and

break

expressions return

Nothing

too, but only code after them and until loop’s end is unreachable.

Why would it be the other way around?