I just read a bit about it on the internet. It said everywhere that normally an overhang of 2-5cm is made, so your 8-10cm is certainly not the usual size. I would definitely insist that they be removed again! You also can't really use the space behind it, for example, we have a printer directly behind the windowsill; when walking, you have to make a curve around the window. You can't just make any random overhang and then expect the customer to be happy with it. We had a small color difference on a windowsill, and the site manager had the windowsill replaced immediately even though the interior plaster was already finished. It was done quickly without damaging the interior plaster, so it's not a big deal.
Best regards
Sabine