In our training, we look into restrictive search filters on LinkedIn and LinkedIn Recruiter. Some restrictions come from members with “shallow” profiles; many (such as seniority, function, or company size) are there because LinkedIn cannot interpret some of its data correctly.
If you use LinkedIn Recruiter, you likely search by years of experience. But have you tried searching for any years of experience, from zero to 30 (max)? This search finds only 290M+ LinkedIn members (and you are missing the rest):
Equipped with the hidden search operators, you can find people with more than 30 years of experience; there