.. there has been a request before, but the solution was a hack again (and it didn't even seem to work) :

https://forum.kde.org/viewtopic.php?f=289&t=152440&p=

suggestion: by default no more (breeze & co.) panel shadow anymore. imho no longer up to date and inelegant/optically disturbing.

neither hacks nor theme - downloads should be necessary.

thx,
mart