I note that this option to 'always show' borders is now gone from "Advanced" Window Behaviour settings.

I am currently experiencing the situation now where verticle window decorations are not seen on maximised windows.
If however I right click the maximise button from a normal window state the window maximises horizontally with the decorations shown correctly.

Anyone?

Is there area where the decorations should be simply empty or blank, or is the window content taking up the entire screen?
Also, can you reproduce this issue with all applications (ie. Kate, KWrite, etc) and under a new user?

Thanks bcooksley for replying,

Window content takes up the entire screen when maximised with only the top decoration shown correctly. If I right-click on the maximise button from a normal window state all decorations show as they should. I've tried a number of different window decorations and all behave the same way unfortunately.

I've tried forcing a smaller window size (1340x748px on a 1360x768px screen) via Special Window Settings but this only results in missing vertical + bottom window decorations.

The same scenarios exists with a new user account.

This image here shows the result of forcing a smaller window size of 1340x748px.
Note the window content vertically remains unchanged also.

That looks correct - although KWin should probably handle the window borders better when the window cannot take up the full horizontal width of the screen. I'll leave it to the developers to comment as to whether this is intended behaviour with regards to maximisation.

The setting was removed from KWin core since it actually never handled that in the first place and what it did was done differently for other conflicts.

Decos are still free to offer maximized border handling:
https://bugs.kde.org/show_bug.cgi?id=324011