I came to the conclusion that:
Any aliasing on a paid mailbox has limits - exploits could connect an alias to the mailbox. So paid mailboxes don’t suit threat models around political speech or avoiding doxxing. (I include forum accounts here.)
Although @dngray pointed out:
That isn’t really possible, because the email is physically forwarded through the alias servers and on to the destination, likewise, the email leaving there same thing in the opposite direction. TLDR not a concern. The only real way around that is lawful interception by the provider, and if that is part of your threat model no email account will be suitable.
I’m still a little unsure. What if Proton itself is breached? All the mail is encrypted 
but I’d guess there are unencrypted records linking the alias to the mailbox to your identity (paying for the mailbox). Add a breach of the forum, linking your forum posts to the alias, and all is revealed.
Of course the likelihood of that chain of events seems low, so if it’s casual discussion (like this forum) I’m sure use the same underlying mailbox is fine. People find out you care about privacy, so what? But if I had strong and unpopular political views (not illegal but with severe social consequences) I wouldn’t feel comfortable. That’s not my situation though, so I haven’t thought about it deeply.