Now spam means "anything the user doesn't want to receive". Even if the user previously deliberately created a relationship with the sender. Gmail successfully trained users into clicking the spam button for anything such. People just click Gmail's magic make-it-go-away button for things as legitimate as a mailing list they deliberately signed up for and are simply too lazy to properly unsubscribe.
Legitimacy isn't a binary either-or, it's a continuum. You bought from a merchant ten years ago and they email you again, is that spam? How about the company that acquired that merchant which is now trying to sell you a completely different line of business? You donated to a politician and now their successor in the same party emails you, is that spam? That's what modern spam filtering deals with.
Spam means anything from someone who is indifferent to whether the recipient wants to receive it and doesn't get "sufficient" permission. I think this is consistent with the original type of spam, even if there is no bright line delineating it.
If you're implying that anything with a working unsubscribe link shouldn't be considered spam, then I think this definitional issue relates to the same controversies about what constitutes consent that have been discussed a lot recently.
I mean, I agree with your last paragraph, so far as that goes, but I would suggest rather than the definition of spam changing, ordinary businesses got spammier/sleazier.
It wouldn't be efficient today with spam being more more diverse and often hardly distinguishable from legitimate emails. A lot of spam nowadays is like forced advertising, by opposition to plain scam and dodgy stuff.
The only things I can think of that I do differently than many people that might make a difference is that I have my mail client set up to not show remote images ever and I never dirctly click any link from email no matter the source. Possibly things might be much worse otherwise, but from my perspective it doesn't seem like spam is a huge issue these days with basic filtering.
An alternative narrative is that previously email servers were supposed to accept almost anything from everyone and spammers took advantage of this. From this perspective it is the relatively recent requirement of sender verification that made the key difference and allows Spamhaus to work (obviously they need to do something to determine who to block). This perspective seems to match what I am seeing.