That would be a significant change in behavior that would require users to view all unread threads before they were marked as such.
I agree that it's a significant change. It's not so significant as you think: There's still the Link
Forum Actions -> Mark Forums read, and with two clicks you have all forums/threads marked as read.
Folks expect to come back after inactivity and automatically find that only threads updated since they were last active are unread.
Maybe, but maybe not. If folks don't have much time to come here and look for new threads, they now have two choices:
* come here and look for the important/interesting threads, but have to leave before they can read all the threads they normaly would - and so "loose" some posts.
* don't come here and look for an occasion later where they have more time to read all the new posts they want to
When you change the behavior it wouldn't matter how much time one brings in when visiting the forum as you can't lose any interesting new posts. So I guess there would be more people visiting the forum spontaneously and so making the forum even more vital. It wouldn't matter if you don't read a thread for three visits, as you can automatically start reading the thread where you left off, even if there are new posts.
By the way: Suggested setting is standard on new vBulletin installations, so the users should get used to the new system really fast, as they could know it from other forums they visit. Speaking for me you are the only vBulletin forum I regularly visit that has this old setting.
I don't know how great demand for this is, but I hope users would enjoy the added usability (and even read more threads
😉). Maybe you can talk about that with the other moderators and if you don't get to a clear opinion for changing it I/you can create a new thread asking the users for their opinion.