For 10 years I've been using a custom email for every retailer I shop at that asks for an email address, always in the form of "company@mydomain.com". I did not keep track of how many custom emails I used (hundreds, easily), but I have received spam from exactly zero of these accounts.
The only account that I received is one I used on my public website as a "mailto:" link. 100% of my spam comes from this address. I host on runbox.com.
Is the fear of "people selling your email to spammers" a modern myth, or are spam filters that good?
I would argue the former since I still get 30 spam emails a day from my website email address, and zero from companies that ask for them.
Email databases for sale are not always for spam or malware. They are often used for tracking and cross marketing calculations. Placing a companies name in the address will signal a canary and they may likely filter your contact out of their database or at least flag it and treat it differently.
I've been using email canaries for decades but recently had to adjust my canaries to be less obvious. A few vendors got upset that I had their name in the address and one even accused me of fraud and canceled my $500 gift card. That was the Tractor Supply Company.
Either way I will continue using canaries and multiple domains as it is a good way to be filtered out of some cross marketing databases and to avoid some behavioral tracking and some machine learning. It is also useful to find companies that get upset. This is an indicator to me they lack integrity and should be avoided. Canaries are also a good indicator to detect if a company has been compromised.