> I think this started around the 90ies that devices turned into magic black box consumables that are expected to "just work" while being undiagnosable when they don't.
I would say that it's more that the architectures where a manual created by the integrator could tell you anything useful, became irrelevant/obviated by architectures where it wouldn't.
Including a manual with a printed wiring block diagram of the hardware, made sense in the 1970s, when you (or the repair guy you called) needed something to guide your multimeter-probe-points for repair of a board consisting of a bunch of analogue parts.
And such a manual still made sense in the 1980s, now for guiding your oscilloscope signal-probing of jellybean digital-logic parts ("three NOT gates in a DIP package" kind of things) to figure out which ones have blown their magic smoke.
But once you get to the 90s, you get complex ICs that merge (integrate!) 90% of the stuff that was previously sitting out as separate components on the board; and what's remaining on the board at that point, besides those few ICs, just becomes about supporting those complex ICs.
At that point, all of the breakage modes that matter, start to happen inside the ICs. And if it's the ICs that are broken, then you none of the information from a wiring block diagram is going to be helpful; no problem you encounter is likely to be solved by probing across the board. Rather, you'll only ever be probing the pins of an individual IC.
Which means that what really helps, in the 90s and still today, are pin-out diagrams for each individual IC.
Providing that information isn't really the responsibility of the board manufacturer, though; they didn't make the ICs they're using. Rather, it's the responsibility of the IC company — who you don't have any direct relationship with, and therefore who don't have cause to be sending you you data-sheets.
Thankfully, these IC companies do sell these parts; and so they mostly have their IC data-sheets online. (No idea how you would have figured any of this out in the 90s, though. Maybe the 90s equivalent of Digikey kept phonebook-thick binders containing all the datasheets they receive along with the parts they order, and maybe repair people could order [photo]copies of that binder from them?)
I would say that it's more that the architectures where a manual created by the integrator could tell you anything useful, became irrelevant/obviated by architectures where it wouldn't.
Including a manual with a printed wiring block diagram of the hardware, made sense in the 1970s, when you (or the repair guy you called) needed something to guide your multimeter-probe-points for repair of a board consisting of a bunch of analogue parts.
And such a manual still made sense in the 1980s, now for guiding your oscilloscope signal-probing of jellybean digital-logic parts ("three NOT gates in a DIP package" kind of things) to figure out which ones have blown their magic smoke.
But once you get to the 90s, you get complex ICs that merge (integrate!) 90% of the stuff that was previously sitting out as separate components on the board; and what's remaining on the board at that point, besides those few ICs, just becomes about supporting those complex ICs.
At that point, all of the breakage modes that matter, start to happen inside the ICs. And if it's the ICs that are broken, then you none of the information from a wiring block diagram is going to be helpful; no problem you encounter is likely to be solved by probing across the board. Rather, you'll only ever be probing the pins of an individual IC.
Which means that what really helps, in the 90s and still today, are pin-out diagrams for each individual IC.
Providing that information isn't really the responsibility of the board manufacturer, though; they didn't make the ICs they're using. Rather, it's the responsibility of the IC company — who you don't have any direct relationship with, and therefore who don't have cause to be sending you you data-sheets.
Thankfully, these IC companies do sell these parts; and so they mostly have their IC data-sheets online. (No idea how you would have figured any of this out in the 90s, though. Maybe the 90s equivalent of Digikey kept phonebook-thick binders containing all the datasheets they receive along with the parts they order, and maybe repair people could order [photo]copies of that binder from them?)