You know that if you edit a file of NodeBB itself it may refuse to update by git pull because a merge would be needed?
So it may be that you're working with an old version of NodeBB o_O
If you are able to delete a post you have the permission to also restore it. Because of this the post doesn't get removed but only gets less opacity (within the official themes).
I just don't know in which case the post could be removed from DOM after reload, may be an inofficial theme or an old version of NodeBB (maybe because of the reason above).
But perhaps I should add a not in the sign up agreement that "by signing up you agree to receive emails from us" or something like that, to be safe.
If you're in the EU, this wouldn't constitute as opt-in. The EU states that a user must specifically click a button that states "I want to receive these emails". Unless it's regarding their account. So, if the daily digest included notifications, it would be fine, but if it was just new topics, I'm not so sure. It should be fine either way, as you've said.