If you are referring to the yellow "payload" sticker that the OP is talking about, they put those stickers on because they have to ever since the TREAD Act was passed in 2000 after the major Ford Explorer and Firestone rollover recall in the late 90's. This is why you never seen them before 2000.
" INFLATION LEVELS AND LOAD LIMITS.—In the rulemaking
initiated under subsection , the Secretary may take whatever
additional action is appropriate to ensure that the public is aware
of the importance of observing motor vehicle tire load limits and
maintaining proper tire inflation levels for the safe operation of
a motor vehicle. Such additional action may include a requirement
that the manufacturer of motor vehicles provide the purchasers
of the motor vehicles information on appropriate tire inflation levels
and load limits if the Secretary determines that requiring such
manufacturers to provide such information is the most appropriate
way such information can be provided."TRANSPORTATION RECALL ENHANCEMENT,
ACCOUNTABILITY, AND DOCUMENTATION
(TREAD) ACT