You may want to know that Ubisoft’s Rocksmith® 2014 Edition will be de-listed from Steam and other online stores after October 23rd, 2023 which is one week from this post date. This is a game that makes learning to play the guitar like Guitar Hero/Rockband, which can score you or let you slowly practice a part with scrolling fret indicators.
Now, it’s not on sale or anything (hasn’t been for a couple years), there might be W11 issues (I’m using Win 10 and Linux computers), and you’ll need a real guitar and either a mic or special USB cable to properly play the game. However, after this date you won’t be able to purchase this game anymore unless you buy from a key-seller or a retail CD copy.
Of course, Ubisoft is going to replace this 40 CAD game with a subscription service model called Rocksmith+ which is 20 CAD per month and not available on Steam. On the other hand, RS2014 works without the need of a Uplay account and can be played offline (Just press Esc twice at the signin screen).
Now there are 1555 DLCs available but they will eventually be delisted as well at some point in the future but you won’t be able to get them without the base game. The only DLC you will “need” is the Cherub Rock DLC because of…
Custom DLC is a community created mod that lets you play user-created and converted maps. By default it uses the DLC ID of Cherub Rock which is why you need it, but this is configurable if you really don’t want to spend 4 CAD. There are tens of thousands of Custom DLC songs available online so you will be hard pressed to run out of new ones to try.
I mean...they're removing it from sale because they have a more egregious business model to sell you instead that no one wants. And that last qualifier you added about alternative channels being illegal is the problem, because we have no measures to preserve things like this.