There were two issues with this:
Incorrect emails would bounce, causing reputation issues over time
I run my own mail server and my forums are "small" so the bounces land in my queue/spool and are not a real problem. I see how this could cause significant grief otherwise, especially on larger forums.
I also use IsTempMail, which seems pretty effective on its intended targets. I wonder how many others do? Been a while since I played with those knobs but iirc, it was pretty painless to set up.
NodeBB prior to this change would allow any email to be entered, even other peoples' emails. A lot of sites do this, and it's not necessarily something we should guard against, but I personally find it really annoying when someone uses my email to sign up for random sites.
Hard nut to crack, this one. Maybe add a disclaimer to the welcome message along the lines of "Somebody, hopefully you, used this email address to register for our community at URL. If this was not you we apologize for the inconvenience."? 🤔