Can't empty the trash after deleting user account
If someone runs into the same problem you need to disable Syestem Integrity Protection before deleting old user folders.
SIP was introduced with Mac OS X El Capitan 10.11 in 2015.


To turn off SIP, one need to start into Recovery Mode.
Turn off the device, press and hold command + R, start the device, and wait until load screen.
In Recovery Mode, click Utilities and start the Terminal application.


Check the status rather SIP en- or disable:

csrutil status


To disable it:

csrutil disable


and to enable

csrutil enable.


Restart the device for changes to take effect.
Start your system as always (not into Recovery Mode).

It is recommended to enable SIP after you deleted the user folder.