Indeed! This is why I stopped using it. I love Fastmail, but who knows if I feel that way in 5 years. The entire point of Fastmail + own domain is never being locked in again. Using subdomain addressing locks you in once again.
I'm with @rb666; Don't rely on it as most will support plus+ addressing but not the the fast mail subdomain addressing as I am now in the process of migrating to Migadu.com and I need to go and unsubscribe and resub using the plus+tag. It's a PITA... lesson learned, stick with best industry practices even if there is an easier method because you'll thank yourself later.
catchalls are great. In addition to allowing the use of arbitrary custom addresses on a whim they make it really easy to identify spam and train spam filters. Anything that arrives on multiple random/unused addresses at your domain is spam.