No joke, the manual for the original pokemon games was one of the best gaming manuals ever created. Not only did it explain everything, but it contained a lot of cool art work that really sold the game world to you. I still remember coming home from the dentist, high af from the nitrous oxide, flipping through the Pokemon yellow manual, and wanting to get home as quick as possible so I could try out my new gameboy color!
Yea, what Tunic did was something that was not quite standard in the "days of yore" IMHO, but at least it was being done here and there. Now you open a case and you have one crappy list of plastic paper with either the basic controls or some "coupon" for extra credits valid only in some crap store. And we don't even have to start about the part how you don't even have the game on the disc most of the time.
But yes, I recommend Tunic to anyone who wants to catch this (and generally that "good ol' times" vibe of gaming) feeling. It's a lovely game and it has that positively tougher difficulty and the need to think about some stuff if you wanna solve all the secrets. Something which not many games do these days (obviously you have FromSoft games and indies but yea).
But this also has a "downside" IMHO. I will be frank, I feel stupid as heck when I think about, say, some of the endings for Sekiro or even stuff in Tunic. I am quite sure I'd be unable to solve most of the stuff by myself. Maybe my brain also got mushed, I dunno. I wonder if I am stupid or if these puzzles are now being made with internet/meta gaming in mind? But I'd rather prefer to not meta game but sometimes I feel almost "forced" (not really as usually this stuff only concerns optional content). I dunno. Anyway, rant over. If anyone wants this kinda vibe gaming used to provide, definitely try Tunic!
Speaking about the back of the case. Remember Metal Gear Solid? Yea, I 'memba.
I will check out Tunic, once I'm done with one or two more games that I have currently installed, but on your hypothesis of games being made with internet knowledge in mind or them just generally being harder: Most games today are (or can be at least) much more complex in their systems than previous generations. Take X4 Foundations for example. It has a properly living economy. As far as I know, no ship and weapon just get spawned in without someone having mined and processed the ressources to do so. The game keeps track of thousands of ships over a volume of tens of thousands kilometers. And since you can mix and match every ship with a huge amount of equipment options, you can't just point at a single thing and say "If I do this, I win." But it also has some obscure systems, I can't deny. For example that you can use EMP bombs to steal building blueprints, so you don't have to buy them.
So while some games absolutely are made with the intent that only those who use the combined knowledge of the internet have a chance at experiencing every secret (looking at you, Five Nights At Freddy's), most games are just harder due to the tech that makes it possible to do a certain thing in the first place.
On the other hand, I remember Morrowind being mentally difficult in some respects because there are no quest markers and very little other help aside from what you figure out on your own and what gets written in the in-game journal.