She’s just been through her junk email folder and found a “We’ve noticed a new login” email from instagram yesterday

The junk-ing security notices is so common.
A few months ago, my dad said “uh, I got some email from my bank, and now my credit card doesn’t work”.
The email was describing some problem with his account which would have been so much easy to fix before they cancelled the card.
Similarly, I lost a domain name because the registrar notifications for renewal ended up in my junk mail.

It’s probably quite a significant issue. Companies can go “well we tried to contact you” and wash their hands.
Doesn’t matter that they also spammed bullshit marketing emails from the same address that issues security/renewal notifications.
Doesn’t matter that spam email has been such an issue it is near-impossible to host your own email server (and expect delivery) for a decade or so now.