The same nothing that does not stop the companies actually seeking their own employees also does not stop 300 recruiters from copying their posted text, stripping out the original company-identifying information, and re-posting.

The noise added to the signal does make it significantly more difficult.

So rather than searching a central repository of job ads for opportunities specifically relevant to you, you have to visit individual company websites, find the "careers" or "jobs" section, and look for leads in a thousand different places.

I consider a no-recruiters policy to be absolutely essential for a job advertisements boards now. Everywhere else, you have to filter out key phrases like "recruiting", "our client", or "resume in Word format" just to find anything useful.