Point of clarification, there's games like Sonic Chaos that have a Green Hill, or Sonic Blast. When we say mainline games, are we strictly referring to the 16-bit Trilogy, (oh yeah and then we've got the modern 2D games such as Mania and Superstars) followed by the Adventure titles, Heroes, 06, Unleashed, Colours, Lost World, Forces and Frontiers? We're not thinking about games like the ones I mentioned initally which were Master System, or games like Sonic 3D Blast? (NOT to be confused with Sonic Blast. 3D Blast was a Mega Drive game, but it doesn't have a Green Hill IIRC and I don't personally count it as mainline, so I'm trying to establish the principle of mainline).
Also are we referring to Green Hill as an explicit level? I don't think Sonic Forces has an explicit Green Hill Zone, but Sonic mentions being in Green Hill (the ever infamous "Green Hill's looking a lot more like Sand Hill right now"), so would we count that from a contextual standpoint? Or because it's not explicitly Green Hill as the level title, that's not necessarily true? Then in the case of Frontiers, iirc Green Hill doesn't appear explicitly as a location. The Cyber Zone levels merely take on Green Hill's appearance, but they're not Green Hill proper.
Guess what I'm saying is, what are we counting as Green Hill in this case? And what are we counting as "Mainline" Sonic?