I think banner is more appropriate behaviour. People tend to close the pop-ups without even reading it. They also tend to skip reading the toast-like banners as well but it's less intrusive. You don't cover the content at the end of the day. Also the first option gives you the chance to tell users why it's coming soon.
Speaking of that, can't deny the fact fake doors give you valuable data but I believe there's a chance that they also can hurt the branding reliability. Fake doors can be described (in a most naive way) is tricking your users.
I honestly believe in quality content over tricks like this. If I put myself in the shoes of a user, I'd love to see details, what's going behind the curtains and learn about why the link that I've clicked is not available yet. Sorry taking the topic to another place. But it really needs to be executed really well and you need to inform your users well while applying this method. Otherwise it's no different than doing this.
edit; also unreleased feature announcements are a great opportunity to collect emails in a legitimate way.