[anthk]

Every modern CL compiler (SBCL, CCL, ECL)... will implement it 'de facto'.

[lispm]

It's just that "it" has no defined standard. See https://github.com/pcostanza/closer-mop/blob/master/features... for the MOP differences on platforms which have support.

Closer to MOP is a library which tries to fix many/most of these differences.

https://github.com/pcostanza/closer-mop