It speaks volumes that channels like Achievement Hunter hasn't posted anything on how to get achievements for a game in years and transitioned to variety stuff and let's plays.
There are still many successful channels that make guides on how to earn achievements and trophies, such as PowerPyx and PS4Trophies.
I think a reason they made the change to more generic content could be to have more control over their content. With achievement channels, your success largely fluctuates depending on which games are releasing, what achievements they have, if people actually need videos about them, and so on. Most of the games that bring in the big views are coming in at the end of each year. If it's just general gaming content, you can freely release content all year round .