This upvoted Money.SE answer claims that it is illegal for 401(k) plans to permit participants under the age of 59 1/2 to make the non-hardship in-service withdrawaldistributions that isare necessary for the mega backdoor Roth conversion to participants under the age of 59 1/2. In my opinion, the answer gives pretty good, but not overwhelming evidence, that this is the case.
(I have trouble understanding why so many people discuss the mega backdoor Roth without mentioning the fact that it's impossible for people under 59 1/2 to complete if, indeed, that is the case.)