When you get a chromebook you can redeem an offer for 100gb free for 2 years. When your 2 years is up, files aren't deleted but you can't upload anything else until you're below the 15gb.
Drive is always free up to 15gb.
Of course, these days the Google Drive storage limit is shared with GMail so you also can't receive emails until you're back below the 15GB limit.
To add to this, adding 100gb back to your account is (just) $2/month or $20/year.
Google Drive is only free for three years? That's news to me. I'm quite certain I've been a user for 4+ years (since its release).
I don't own a Chromebook though. Maybe that would change things.
Unless it's different with Chromebooks, it's free up to 15 GB.