DOF CoC is not about megapixels, it is about sensor diagonal size, and the degree of viewing enlargement expected. When we used popular 35 mm film, the common estimate was CoC 0.03 mm. Because, the 35 mm film diagonal is 43.3 mm, and the estimate for viewing the standard definition of an 8x10 inch enlargement (viewed from a distance of 10 inches) was diagonal / 1442, which gives 0.03 mm. This being the dot size a human eye might be able to see (to perceive an enlarged dot size seen at 10 inches), and DOF computes when CoC and enlargement becomes that size. The 0.03 mm only applied to 35 mm film size (the sensor diagonal). Also full frame size DSLR today (also 36x24 mm), and for the popular APS-size sensors (24x16 mm), it becomes 0.02 mm, and still assumes viewing enlargement to an 8x10 inch print.
In past history, there were other estimates for CoC, diagonal / 1732 in the beginning, and later on more recently, diagonal / 1500. However, the 0.03 and 0.02 mm CoC numbers became popular with Japanese camera manufacturers, and that is an implied divisor of 1442 (used by virtually all on-line DOF calculators).
The smaller the sensor (needing greater enlargement), or for greater enlargement than 8x10 inch, this enlargement allows visually seeing the blur better, so the CoC limit has to become smaller (to still recognize the blur the same equal way). Or instead, if you view a smaller image, or have a larger sensor, CoC can become larger.
Depth of Field calculators must ask which sensor size to compute usable CoC, but the DOF standard still assumes viewing 8x10 inch size at 10 inches (rarely mentioned today). I have one at https://www.scantips.com/lights/dof.html that also asks print size (enlargement) and also provides for different divisors.