Please don't assume that the Lisp compiler on some Symbolics could not output micro code. It could, IIRC.

But it was not what a Lisp developer normally would do, he/she would use the compiler in such a way that it outputs the usual machine code, not micro code.

Computer Architecture is not defined by what a Lisp developer normally would do.

What hardware features of the CADR do you feel provide support for GC and tagged words ?

I have built the CADR microcode from the same source to use both 24 and 25 bit pointers, it is just software.

Whether microcode is hardware or software is blurred. Remember, when microcode was introduced in the 1960's, it was used for implementing the same thing in software that other versions of the same computer family did in hardware. With microcode, a vendor could offer different machines at different price/performance points. A sequential circuit can implement an algorithm; microcode can implement an algorithm.

The CADR is an example of a computer that implements a virtual machine in software, what makes it special ?

Computer architecture on the user level is defined by the data format and instruction set the CPU offers. How it is implemented is another level. I don't know how some Intel i7 is implemented, but it probably has writable microcode and some very different architecture inside.

That Intel hides the microcode and the CADR didn't is just another detail.

You still haven't identified what hardware features you think can be emulated in software on a CADR but couldn't be emulated in software on a 68020.