Fortunately MovieChat.org took up archiving as much as it could of the old content, and up to today providing the same 'style' of community engagement as the old boards. I wonder if they'll do the same for User Reviews.
There's even a userscript [2] that seamlessly puts an embed of the equivalent MovieChat board (if available) for each IMdb entry, right in the site section where the old IMdb boards used to be.
I assume you can make an account on MC and still participate in all those (their) boards.
I'll admit though-- probably like many, I came to the old IMdb boards just before the deletion, and to be honest they were kinda shitty. Sorry. Alot of trollposting and lack of seriousness when someone tried to have a serious discussion about some work. But it was overall and mostly Fun!
The MovieChat boards still have much of that old content - take a look and enjoy.
[1] https://wikipedia.org/wiki/IMDb#Message_boards
[2] https://greasyfork.org/scripts/27617-new-imdb-message-boards...