I switched to using masked emails with Fastmail primarily so I could see who sold my data. The potential security benefit was not really a driver. Having 1Password be able to generate a unique email makes it a no-brainer these days. For those services that require a username that is not your email, they can usually be used without the domain part. Works really well.
I even wrote a tiny little local only web app that I can use to generate a masked email on my phone, so when I need an email for an in person thing I can just show them my brand new weird email directly on my phone.
Not really any places where things get sold, but opt-in in the background for newsletters is bad in certain sectors. Ticket platforms are terrible. I like to use a new email for every event and boy does that lead to new round of clicking opt-out until I can deactivate the email after the event has concluded.
I even wrote a tiny little local only web app that I can use to generate a masked email on my phone, so when I need an email for an in person thing I can just show them my brand new weird email directly on my phone.