the general place for changelogs is https://documentation.kopano.io/kopano_changelog/webapp.html.
But this place is only ever updated when we produce a release for our customers, therefore this hardly ever matches with the nightly builds available from the /community page.
What you could do though is the following:
# check out the webapp git repo (this is publicly accessible and does not require registration
git clone https://stash.kopano.io/scm/kw/kopano-webapp.git
# change into the checkout
# print out all the changes since a certain release
git log v3.4.6..HEAD
so the above will get you all changes that have been done since the 3.4.6 release has been tagged and release.