If you change the keyboard repeat delay in systemsettings5->Input->Keyboard and press "Apply", the setting is not applied immediately (neither in the test field nor in other applications).
Logging out and in again is necessary to actually apply the settings.
This is on X11 and it apparently is a regression in Plasma 5.18 (so it's not a duplicate of bug#410626 IMHO, which is older and about Wayland).
I tried on an openSUSE Leap 15.2 LiveCD too (which is a snapshot about a month old and still contains Plasma 5.17.5), and it works there.
Likely related, but I forgot to mention it: the keyboard repeat speed has the same problem.
Git commit 27a64b96f224b510234c76c22bcc6407a86ea4b8 by Weng Xuetian.
Committed on 18/12/2020 at 06:08.
Pushed by xuetianweng into branch 'master'.
Reparse the key repeat rate config when we try to load it.
Right now due to the fact that it is a shared config, we may load the
old value from memory.
M +4 -1 kcms/keyboard/keyboard_hardware.cpp
*** Bug 430062 has been marked as a duplicate of this bug. ***