Might as well use this downvoted thread for my little hobby horse as well: I was done with Fastmail when I realized they recycle email addresses if you stop paying.
They should either lock access to the account until you buy a new subscription (which seems obvious, just hold the account hostage), or lock the account forever if they don't want to store/recv messages for inactive accounts.
This is also true of every personal domain used for email. While it's not great from Fastmail, it's no less secure in that respect than a custom domain.
Also I mostly use mail via IMAP using the iOS mail app, in which case the web client is entirely irrelevant.