Clarify that GPC signal does not legitimize cross-site tracking as default web condition

[j-br0]

At TPAC, some commenters noted that because GPC is an opt-out signal, standardizing it could be viewed as an implied support for the position that cross-site tracking should be allowed by default. That was not the intent of the creators of the signal; rather, it was designed to allow people in opt-out based regimes to exercise their privacy preferences at scale. It was not intended as an endorsement of opt-out based approaches.

To address this concern, we should add language to either the spec or the explainer clarifying that standardization of GPC is not intended as endorsement of either tracking by default or opt-out based legal systems.