I gave an answer to this question but user ptityeti down-voted it mentioning:
-1 Manaslu is a clear example where it is required. I've heard of plenty of people that they where stopped at the Jagat checkpoint for not having a guide. I've never heard of anyone being able to enter there without a guide. Because that particular law doesn't say anything about it does not imply that there is no other regulation making it mandatory for some national parks.
And the problem with that comment is that Manaslu is not even a national park and is a conservation area, and the answer I gave is only for national park and wildlife reserve.
I am asking this question because that comment has two upvote, so am I missing anything? And I know we are here for Q&A not for reputation but for new user shouldn't down-vote be justified.