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.
https://www.fastmail.help/hc/en-us/articles/4406536368911-Ma...
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.