i also liked very much the new "launch" page - https://www.phpbb.com/about/launch
one small nitpick: i think the "Over 1k bug fixes, 1.5k improvements & 11k commits." on top of the image emphasizes the wrong thing. counting the # of bug fixes, or the number of commits is just backward.
if you think emphasizing changes to the code is a good thing, i'd rather see something like "45K lines of code deleted" or something.
another code-related change that i would not mind emphasizing is the change in direction, represented by heavy reliance on 3rd party open-source packages - symphony, composer, twig, jquery, etc.
(this is a very welcome change of direction for phpbb - in the past, similar suggestions, e.g. to replace the in-house templating system with 3rd party systems such as "smarty", which uses almost-but-not-quite identical syntax, were rejected by the then-developers offhand. i like the current dev-team attitude much better).
again, this is just nitpicking - the main thing here is - congratulations!
i believe that despite the seemingly "minor" upgrade represented by the change from 3.0 to 3.1, this is a quantum leap for phpbb, easily as significant as the upgrade from 2.0.x to 3.0.
as a side, in the annals of personal computer software history, "version 3.1" is almost magical - so much so, that micro$oft, when releasing Windows NT, marked the very first released version as "Windows NT 3.1" instead of 1.0, hoping that some of the magic will stick. it didn't, but i have a feeling that it definitely will for phpbb.