Valve has updated its rules about in-game advertisements. Developers can no longer lock content behind one or hand out in-game content in lieu of watching an ad. https://www.notebookcheck.net/Valve-updates-guidelines-to-reign-in-predatory-advertisement-practices.959232.0.html
"Then again, developers engaging in such practices can always look at other platforms or self-host."
Good. Let's keep Steam free of trash devs & as good as it has been. I, for one, wont be installing other PC store. Steam makes it pretty simple to sell and buy without installing unknown stuff (right, Epic?).
"Unfortunately, developers can still advertise products in game. Death Stranding did this tactfully with Monster Energy, while EA was a lot less subtle with full-screen ads in UFC 4. Thankfully, this trend never picked up, but if it ever does, it is perfectly okay by Steam."
This wouldn't be a problem if an update came in and put the ad, and another update came and removed it from game assets. But I'm pretty sure they are doing it a la live-service way and collecting data and whatnot from users. Some old games had sometimes tie-in brands inside as advertisement, and it was actually cool at the time.