From the changelog [1]:
Some of the fixes included in this release are:
* kio-fish: Only store password in KWallet if the user asked for it.
* The Umbrello Fixes for adding multiline c++ comment support.
* The scrolling behavior in the Okular document viewer has been
improved and is more usable with free-spinning mouse wheels
* A regression that sometimes caused the JuK music player to
crash on start has been fixed
* The Kdenlive video editor has received many stability updates,
including a fix to the DVD chapter creation and a fix that
improves the handling of timecodes, improved handling of missing
clips, draw “photo” frame on image clips to differentiate from
video clips., and previews in the timeline
* KMail now correctly handles existing maildir folders when adding
a new maildir profile and no longer crashes when adding too many
recipients
* Import and export of Kontact settings has been enhanced to include
more data
[1] https://kde.org/announcements/changelog-releases.php?version=20.04.1