I'm sorry if this is wrong palce to report this bug, but I wasn't able to find better. I mean service placed in "System Settings -> Background Services -> Startup services" with name "Free Space Notifier", which seems that doesn't work (at least for me). I'm not sure what is the minim limit of free space on disk when mentioned notifier will show the warning message. Also I don't know how to configure it (I'm not sure where should I find properly dialog), probably this is placed in some configuration files in /usr :/. I remember that in KDE4 I was able to configure this limit. In Plasma 5 I sill wasn't able to do it. OK. I assume that the limit is 200MB. I had several hours below 200MB free space on root partition and don't see any warning message. Now I have 133M couple hours when I turned on computer. Of course mentioned service is turned on. I think such waring is quite important. It is not so nice to be surprised by package manager saying that user has not enough free space to update the system :/.
The free space notifier warns you about running out of space in your home folder, not /. Perhaps it should do that too.
Or maybe should warn about / and /home or all mounted in local disk. Anyway would be nice if user would be able to configure (by GUI) these limits and alternatively checking disk/partition.
*** This bug has been marked as a duplicate of bug 340582 ***