Hmm I cant post links, but if you go to degiro's website then click on "about degiro" then "safe and reliable".
You can see it mentioned here under "Interest Received".
"Interest is received on money held within the 'Cash Fund'. For the EUR Cash Fund, this equates to the Eonia with a maximum deduction of 0.50% on an annual basis. Eonia is Effective Overnight Index Average and in the past ten years has moved between 4.3% and 0.05%. In the worst-case scenario, you will receive zero interest as a result of this deduction."
They also confirmed it to me in an email.
"Any tax on this interest would not be deducted automatically and would have to be declared yourself."