The Let's Play-- also known as "Let Us Play" is the general idea of a video game playthrough in either a forum post with screenshots, or even a video of the game with commentary.
The Let's Play concept originated on a website called "The Something Awful Forums" on a section called "Let Us Play" hence the name. Since posting on the website was (and still is) paywalled, the early adopters of the Let's Play format were paying to post their series on the site.
Upon the few first Let's Play's posted on YouTube the niche started becoming more approachable, more and more creators started making Let's Play videos. Nowadays, most of the original adopters are still posting Let's Play videos to this date.
Since YouTube has had a lot of changes over the years, and the market has become saturated, the Let's Play niche has died off, though many are still attempting to gain a following in that space.
The saturation of the niche has made it virtually improbable to get noticed, though there are a few that still break through.
A few notable Let's Play content creators that are still posting today include (off the top of my head):
Despite the Let's Play being saturated enough to almost guarantee a dead channel, there is still hope for those that put their own twist on the niche. Finding your own style to narrating your videos and changing up the formula of the niche could potentially help you break through the saturation.
There is still hope.