I have made the PR for PHPBB 3.3 support. Actually, I think most (or all) of them are fixes of things that could have been happening in previous versions as well.
I have performed a number of different imports with this code so it’s working for me. However, specific setups could still be triggering unaddressed features or conditions.
There are some fixes with respect to polls and there is one feature / fix that affects personal messages.
Personal messages can have users and groups as recipients, they are coded as u_ID and g_ID. Previous versions of the importer assumed everything was a user and just threw away the u_ prefix. If the recipient is a group, this was resulting in a confidentiality issue, since all messages directed to a group with a certain ID were being imported for the user with the same ID. The group ID is now detected and expanded into individual users. My first idea was to use group inboxes but we came across messages that were being sent to three users and two groups at once, so that made it impossible to use group inboxes. If a message is now sent to a large group, this can potentially result in a PM being sent to a relatively high number individual users though.
The PR is merged now and you can find it here.