A few things I have noticed... phpBB2 is allowed, but must be kept at the most current version. This means you need to check to see if there is a new release on a regular basis and update right away when a new version is released. Fantastico does not always contain the most up-to-date version since Fantastico has to release a new "package" as well to include the updated phpBB2 files. So, for the time between the release of the new version from phpBB and the update of Fantastico, phpBB2 is not available in via Fantastico so people can not install the out of date version of the script, which would then be disabled. Once Fantastico has released a package with the updated phpBB2 it is made available again.
The forums here were switched to SMF because it was viewed as easier to maintain and keep up-to-date. (Not necessarily because it has fewer updates, but because the updates are easier to apply.) Given the issues caused by out of date phpBB installations, a desire to protect their customers, and several other factors the staff at LP recommended SMF as an alternative to phpBB. To the best of my knowledge, at no time did LP say they were banning phpBB, that people HAD to convert to SMF, or any similar comment. Some read more into LP's suggestion to convert to SMF than was really there, especially with the disabling of out-of-date phpBB installations for security and server stability reasons. I believe some saw the suggestion to conver, found their phpBB installation disabled because it was out-of-date, and incorrectly assumed the phpBB had been banned rather than realizing all they had to do was perform the update properly. The various versions, third-party installs, and other mods to phpBB have caused some problems where the autodetection script incorrectly identifies an up-to-date version as being out of date. These issues are unfortunate and I believe LP is working to improve the detection script to avoid the false positives in the future.
So, more than just an answer to your question, but I wanted to throw out what I know since I know there has been some confusion on this issue.