Sorry for the late reply. The behavior changed after updating to XFCE 4.20 with the current testing update. With XFCE 4.18 my preferences to “never” switch the displays off were respected without enabling presentation mode. Enabling presentation mode feels like a workaround to a behavior change or a bug to me.
I assume this might somehow be related to the major changes to lock screen management mentioned here
There is no dedicated “security” tab anymore. Lock screen management was massively simplified and “Light Locker” was dropped. “lock-on-sleep” is now synchronized with xfce4-session and xfce4-screensaver.
Screen locking settings are now only handled by xfce4-screensaver to avoid conflicts. A button to open xfce4-screensaver-preferences was added
Please feel free to keep the topics separate if you feel this to not fit the update thread.
I should have made the behavior change more explicit in my initial post.