I run a service called Blockmetry [0] that measures exactly that, directly from pageviews. Some numbers get published regularly [1]. The percentage from August (last public number) is 5.2% of non-bot JS-enabled pageviews did not fire the analytics tag.

The short answer is that it's significant on an aggregate level worldwide, but the reality is that it varies _massively_ by country, device, day of the week [2], and even different sections on the same site. Additionally, there is small percentage of pageviews that have JS disabled you have to account for. This analysis was on HN earlier today [4] saying 0.2% of pageview worldwide have JS disabled, but, again, with huge variation (notably, Tor, but elsewhere too).

Q4 numbers are not released yet, but the trend is generally up, with some notable drops. Get in touch if you want more info or to set it up on your site [5].

[0] https://blockmetry.com/ [1] https://blockmetry.com/weather [2] https://blockmetry.com/blog/weekday [4] https://blockmetry.com/blog/javascript-disabled [5] https://blockmetry.com/contact