then some steam games would theoretically also be exempt because they don’t use steam drm.

I think the main difference that would arise between these and GOG would be the provision of installers. Even though some Steam games don’t use its DRM, they’re still reliant on Valve’s servers and an online connection for installation. GOG games are reliant on CD Projekt’s servers and an online connection for installer downloads, but upon download completion, one may install and reinstall games even while offline.

That’s a critical difference in digital distribution, in my opinion.

But isopropyl alcohol and enough elbow grease will get it off, if it’s just a coating on plastic.

Do beware, however, that you may want to dilute the alcohol to some degree, or simply use a lower concentration form of it. Too strong and it may eat at the underlying plastic just as much as the coating and ruin it.

If the Otterbox case had a rubberized coating on it to try to improve grip, and with it being 6 years old, there’s a possibility it’s the culprit. You could try ditching the case for a little while, and/or getting a new case and swapping them out, clean the surfaces again and see if you feel the stickiness again after handling your phone and other stuff.

However, often with those rubberized coatings, the degradation (when severe enough to feel sticky) is more immediately apparent and you’d be more apt to avoid touching anything else afterward. Also in my experience I don’t recall it transferring to other surfaces much, but then again when I dealt with it I noticed ASAP and cleaned my hands right away.