I can believe that about "since the 90s" (per the comic,) and I could believe it about particular fields that may operate weirdly. But I have pretty diverse interests and have never had any difficulty finding things on SciHub or Book4You (which may represent a more recent development.) I've had trouble with particular books, almost always ones that either just came out or came out prior to pdf proofs, but not subjects. So I would guess that it's a minority of fields, granting that I might be exaggerating how worldly I actually am.

But if someone knows their own field (or just a field they have an interest in) isn't generally available I would certainly be interested to hear that! (If disappointed for their own sake.)