When a method has a non-void return type, you're responsible for ensuring that all code paths are terminated by a return statement.

In this case, your switch statement prescribes the return value for a specific set of logic paths, but the compiler can't guarantee that the condition will be true. You need to provide a return statement (it can be return null;) to ensure that a return value is provided should your condition not execute. This is true regardless of the logical construct being used (it's not unique to the switch construct).

With switch, you can either place a return statement at the outermost scope of the method (after the entire switch construct), or you can include a when else clause to cover all cases not otherwise expressed in when blocks:

when else {
    return null;
}

When a method has a non-void return type, you're responsible for ensuring that all code paths are terminated by a return statement.

In this case, your switch statement prescribes the return value for a specific set of logic paths, but the compiler can't guarantee that the condition will be true. You need to provide a return statement (it can be return null;) to ensure that a return value is provided should your condition not execute. This is true regardless of the logical construct being used (it's not unique to the switch construct).

To use a simple example, the following is not allowed, because the compiler cannot guarantee that a return value will be provided:

public static Object retVal(String input) {
    switch on input {
        when 'Test' {
            return true;
        }
    }
}

This is one way to make it legal, using when else to cover any case not already handled by a when block:

public static Object retVal(String input) {
    switch on input {
        when 'Test' {
            return true;
        }
        when else {
            return false;
        }
    }
}

That guarantees that there will be a return value, regardless of the logical path followed and regardless of the input parameter.

Another way to deal with the issue is to place a simple return statement after the switch. This applies equally to other logical constructs.

public static Object retVal(String input) {
    switch on input {
        when 'Test' {
            return true;
        }
    }
    
    return false;
}