Chess Mentor was released in 1995 and had a fair amount of success. Unfortunately, the company that developed it went bankupt in 2000 and the rights to Chess Mentor were acquired along with the domain name Chess.com in a bankruptcy auction back in 2005. (The total price for everything was $55,000!) Many of the old lessons date from the 1990's.
You can actually follow the timeline of Chess.com on the Wayback Machine from the 1990's if you want. The very earliest crawls are mostly empty. A good one is https://web.archive.org/web/20000611102405/http://www.chess.com/
The plan to eventually eliminate "some of the old lessons" was announced in January 2019 by Erik.
What is happening to the old lessons (aka "Chess Mentor")?
Speaking of "old lessons," yes, that's what we decided to call them. It was the easiest term. And all of our old lessons will continue to be available (from links on Chess.com Lessons, and at www.Chess.com/mentor). Eventually we will bring over the old lessons into our new "mastery" section, along with a lot more content.
What is happening to videos?
Our videos section will continue to be there. A lot of the content will come over to the mastery section of Chess.com Lessons, along with new challenges to accompany the videos. New content will also come to videos, but with less emphasis on learning, as that will happen primarily in Chess.com Lessons.
Old lessons will exist as they do now until we can bring that content over to Chess.com Lessons. In many cases that will mean adding videos, or updating the content. Some old courses will eventually go away.
I don't get why you want back lessons you have already done, is not having new content better?
Many of the old courses had much more material in them than the newer ones. And every educator knows that repetition is a key component in really mastering material.