This is known as "cognitive accessibility" and often neglected. MDN has a good writeup on it and mentions ADHD specifically.
https://developer.mozilla.org/en-US/docs/Web/Accessibility/C...
https://developer.mozilla.org/en-US/docs/Web/Accessibility/C...
It's not like I'm looking away from my monitor every 5 seconds and then completely forget where I am or what I was doing. Understanding the content is also not an issue.
The real problem is the insidiousness of the advertisements themselves.