Yes, i am against adding immutability bit in VM.

But if we going that way, then please explain me how VM will respect
immutability and #become: or #becomeForward:
at the same time.

The problem is following:
suppose your object (a) is referencing object (b).

now you doing:
 a beImmutable.

and then

b becomeForward: c.

which leads to replacement of reference to (b) by reference to (c)
in an object (a), which is immutable.

You could say, lets ignore this.
Then, apparently , expect to see a hacky code, which as a workaround
of immutability, using #become..

-- 
Best regards,
Igor Stasenko AKA sig.