Question: After update, old Eclipse plugins remain in "plugins" folder (there are also leftovers in "features" folder).

Is there a way to remove those automatically?

Answer: 
To get rid of the old plugins you have to clear the Installation History. Go to Help | About Eclipse | Installation Details | Installation History and select all the old stuff. Then click delete and restart. Voila. This works with Eclipse Helios (3.6)
For STM32CubeIDE, a popular Eclipse-based IDE for the STM32 microcontroller-family, this command did clean-up old plugin versions under Windows 10:

```
"C:\ST\STM32CubeIDE_1.3.0\STM32CubeIDE\eclipsec" -application org.eclipse.equinox.p2.garbagecollector.application -profile STM32CubeIDE

```