I tried this, but the thing I didn't like about it was that it ties you into Fastmail. I like using a custom domain so it's relatively easy to change your email provider, but masked emails are in the format xxx.xxx@fastmail.com so you'd need to revisit all of those accounts if you want to change your email provider to someone else. I'm currently trying using a catch-all with servicename@mydomain.com, and will be interested to see how much extra spam comes in through the catch-all.
I do this with a generic-sounding domain (...mail.tld), and I purposefully don't have catch-all enabled for this domain. That way, when I disable a masked address I completely block the service I used it for from sending me emails.
To further reduce ties to Fastmail, I believe you can export a list of all your e-mail addresses used.
I get that. If (and when) I change email providers it'll be a pain. My gripe with a custom domain is that it'd uniquely identify me. I prefer to hide behind fastmail.