That's what I was looking for, but is it possible to change it through configuration? I can see the field qty_delivered in the Sales Order Line model and tracking property, but it's not editable. The Stored property is editable, but Tracking is not.

Now the weird thing is, that if I look up the qty_received field in the purchase order line model, is loooks the same: Stored and no tracking. But changed do get saved in the chatter anyway.

I went through the source code and found that there is a method '_track_qty_received' and a template 'track_po_line_qty_received_template' which enables the observed behaviour.

To answer the question "if it is possible to configure?" answer is no

Unless you're strictly against customisation it's better to do it that way.

If you need help in writing custom code to enable logging of qty_delivered, feel free to reach out.

That's a little bit disappointing, I thought something like this would be part of the base funcionality.

About custom methods: I am not strictly against it, but we're on Odoo online. I understand it's not possible to install custom modules.

You're right, can not install custom modules but you can still use automated action paired with python script.