This is an intellectually indefensible viewpoint as stated, and indeed the source of many of our gravest issues.

For example, gold melts at 1,064 °C (at standard atmospheric pressure). This is statement that is true and accurate to the very best of humans' knowledge. Opinions on this fact are worthless.

The reason your idea is so pernicious is that it allows individuals to pretend that the great issues of our present day don't exist, and simply to deny factual truths as relative, contextual and subjective.

Someone told me about the ongoing climate disaster, 'Well, that's just someone's opinion."

No possible argument can refute this! If you believe your casual, completely uninformed opinion has exactly as much value as 150 years of peer-reviewed science, then there is literally nothing I can say to change your mind about anything at all.

Also, it's perplexing you describe the post-modern viewpoint as somehow twenty-first century, when all the ideas you are discussing were developed between 1940 and 1960!