They often break compatibility in subtle ways to push people into buying newer gear, even though they have deceptively advertised compatibility as a major feature for many decades. It's always been a spotty record of compatibility across generations of products. It's better than nothing but if we all have to buy into the new system anyway eventually, why then make the impression that compatibility is a priority for Nikon? Either it is or is not, if compatibility is not guaranteed then advertising it can lead people to make incorrect purchase decisions that hurt them later on.
Nikon's reputation for backward compatibility rests on a single decision in the mid-80s when AF appeared and a lot more electronic communication was needed, and Nikon decided not to do what Canon did in 1987 and change its mount. Whether Nikon's was a choice to favour backward compatibility over electronics or backward compatibility was invented retrospectively as the excuse for a dumb mistake I don't know, but I suspect the latter is more likely.
I have never seen Nikon advertising that made or implied a guarantee of backward compatibility. In the 1990s, soon after Canon had spectacularly trashed backward compatibility, Nikon advertising made a point of saying that any F mount camera could use any Nikon lens made after 1977. But it was and has never been a major selling point because then, as now, it was innovation and new features that sold. Nikon's Press Release for the D1, eg, has 500 words on sensor, exposure, 4.5 fps etc, etc, and 13 on backward compatibility: "Its innate flexibility allows D1 to accept more than 80 Nikon F-mount lenses"; in the D3 press release there is not a word about backward compatibility; in the Z system press release all it says is "the new mount adapter will enable compatibility with NIKKOR F mount lenses".
In the digital era backward compatibility has been the exception rather than the rule. The vast majority of the dSLRs Nikon sold - D5600, D5500, D5300, D5200, D5100, D5000, D3500, D3400, D3300, D3200, D3100, D3000, D80, D70, D60, D40, and D40X - are not AF-D compatible. On the same list of cameras, Ai lenses can be used but exposure is manual only. Backward
incompatibility of new lenses with cameras even a few years old was usual, even at the high-end: E diaphragm lenses, introduced from 2014, won't work on a D1, D2, D100 or D200, made until 2007; AF-P lenses, introduced in 2016, do not AF on F6, D1 series, D2 series, D40 series, D50, D60, D70 series, D80, D90, D100, D200, in the case of the DX lenses D300 series (made until 2012), D700, D3000, D3100, D3200, D5000, D5100.