Even the Linux kernel qualifies as toxic proprietary code for the simple reason that GPLv3 is not backwards compatible. If anything the GPL is toxic to any other open source license including itself. Others at least try to play well with each other.
That is not true. Being "GPL compatible" is not the golden rule by which software is considered Free Software; granting the "4 user freedoms" is.[1] Since the GPLv2 grants those freedoms, it is Free Software.
Edit: I am not arguing from an ideological standpoint, just from the resulting compatibility and usability issues. The GPL 2 and 3 were both written to protect the four freedoms, that they are incompatible and that we now have to invest time and effort work around that even for free software is just ugly.