Any sources on that? I’m guessing the 90% is hyperbole, but I still wonder how much spam the average user actually gets. (For me, close to absolute zero)

It has taken me over a year to wrangle my inbox into shape (unsubscribing from all marketing email that isn't spam/gets through google's spam filter) and I would say anecdotally that my incoming email has decreased by at least 90%. I go multiple days without receiving emails now.

Pretty much every company/service out there opts you into spam unless you proactively opt-out of it (which takes effort and knowledge to work around the dark patterns).

Looking at my password manager I've got ~260 logins right now and keep in mind that I don't do social media and try to avoid creating accounts as much as possible (and delete the ones I don't use for a long time), so the average user is likely to have a lot more accounts.

Even if each one of these companies only spammed once a week (most will do more frequently if you let them), that'll already significantly outnumber the amount of legitimate e-mail I receive.