This has happened exactly once to me, and it was the VPN, and not the email address.
Paid plan folks can also make use of simplelogin.io
They also expand your storage every year, so it’s not like it’s stuck there forever. For reference, I’ve been on Proton for about 3 years now (paid plan) and I have a data storage cap of 540GB and I’ve never had to buy more. Also, I all my emails so far only consume 340MB - so even on the free plan I’d still have years to go before I reached even 5GB.
(Also, I’ll admit I don’t email much.)
And sometimes they don’t. You gotta check ahead, unless you’re going there anyway