I see the need for this, but I would just ask that there might be some flexibility allowed-for in appropriate cases.
e.g., sadly, I must sell my Orange E4 some time soonish. It is a model year 98, but the frame was built early in 97 so I imagine it was probably first sold in 1997. If given the choice, I should like to list it on both the 97 and 98 forums, simply because otherwise somebody who would like to buy it might not see it if they only look at one of the two forums. I would in any case contact a moderator in advance to seek advice/permission.
I doubt whether there are all that many on-the-cusp cases like this, so if there was a bit of flexibility I hope it wouldn't detract too much from the purpose of the new rule.