Should I leave ZRAM activated on a device with 512 MB RAM or not? I have an old Android Smartphone (Samsung Galaxy S Plus i9001).
It has a 1 core 1.4 GHz CPU and 512 MB RAM, OS is Cyanogenmod 4.4.4.

It runs smoothly and quite fast. The phone will only be used for phoning and a litte bit of photo. (Not quite smartphone useage but there are no classic phones anymore)

The kernel (CM11 kernel) comes with ZRAM activated. The description in Trickster Mod (Tuning tool) says about ZRAM: "reserves 10% of your device RAM as compressed swap space".
Free space after boot, without opening another program is about 150 Mb.

Question:
Should I leave ZRAM activated on a device with 512 MB RAM or not?
Leaving ZRAM on may actually be beneficial to your little S Plus if you leave it on, though your limited use cases (Phone and Camera only) may not see a huge benefit over not having it on. The cache exists to keep commonly used programs, files, and useful bits of data on the memory instead of stored in a page file - this can speed up your phone a little and increase the lifespan of the device.