I am using plasma 5.3 and when applications are launched, they often appear below other apps and do not take focus. I remember the focus stealing of apps being an annoyance, especially when I was trying to type in keyring passwords, etc.

Is this a bug---that is should applications you launch be displayed on top and be given focus? If so, It would be nice if they wouldn't be given focus if the user was in the middle of typing, like if a keypress happened in the last 250ms.

https://bugs.kde.org/show_bug.cgi?id=110543
https://git.reviewboard.kde.org/r/110919/

I've that patch ported for KF5, but there're other patches pending for review (and I don't want to dump my 25 local patches on Martin en block

There might also be different in how apps are launched, because the launcher may steal back the focus and if there's no startup info (like when calling from konsole) the times are messed up.

Maybe your patch solves my problem as well: Krusader, minimized to sys tray; it is always restored in background and sometimes I have to minimize all other windows to show it (nothing happens on click). I'm looking forward the next updates.

Probably no, rather "make it worse".
This sounds as if the SNI button does not (forcefully) pass the focus to the new krusader window (which is required because the focus is likely on the panel/desktop, being another window group, ie. "application")

The patch would however likely allow the window to raise, because I split that linkage (ie. windows may become "visible" but not get the focus - unless they'd cover the active window "too much")