There are two ways to look at this and that depends on how the forums are used.
On a positive note, it could allow the sharing of information to the same level as an email to committee members would. It could also enclose any communications within the GDPR blanket rather than sending personal information by email to private email addresses. Members of the forum could be removed and therefore would lose access to this personal data once they had no right to access it.
On a negative note, you could find that decisions are made outside of the committee structure and you are right, it is not transparent if that is the case.
The forum would have to be very heavily administered by the Clerk and any hint of decision making would have to be removed. It would, I believe, also fall under FOI if a member of the public requested it and would have to be used with this action permanently at the back of people's minds... Therefore no slating off members of the public or gossiping!