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While tracker-blocking in today's sense would have made no sense, there certainly was ad-blocking and cookie-blocking before the 2000s.

AdSubtract advertised it's ability to block doubleclick.com cookies in March of 2000: https://www.computerworld.com.au/article/91102/adsubtract_bl...

The article also mentions Siemens' WebWasher product which blocked cookies. Other cookie-blocking products were released in the same time period.