Multi-res and Alt-D instances

In 2.49, it was possible to sculpt with a mirror instance to increase poly count.
In 2.55, it doesn’t work. Is this a bug?

Doesn’t anyone use this technique?

this can’t work because in 2.5 multiresolution became a modifier and modifiers are linked to objects, not object data. When you create an instance, you create a new object using the same (mesh)data as the old one but not it’s modifiers or anything else sitting on the object level.

btw: in 2.5 sculpt performance was greatly improved so you shouldn’t need this technique anyway