Going back on topic, Apple offers mail hosting for custom domain, along with the optional “one mailbox, infinite aliases” feature we’re discussing right now. I’ve been using it for a while now and no bot spam. No spam at all actually.
They’re not exactly the same thing. Aliases are alternate addresses (under one domain) you set up yourself, while a catch-all just accepts any address in the domain as valid. While iCloud Mail does allow catch-all¹, aliases are capped at a maximum of three².
¹ https://support.apple.com/en-gb/guide/icloud/mm9e3ee0680f/ic...
² https://support.apple.com/en-gb/guide/icloud/mm074af79454/ic...