Apple Support initiated screen sharing with my MacBook running 10.14.4 event though no checkbox in Preference -> Sharing is checked. Didn't find anything so far.
To take back control of your screen—but still let the other person view it—click the Screen Sharing button in the menu bar, then choose “Allow [name] to control my screen” to remove the checkmark.
No. We were on the phone and he asked if he may have a look on my screen. Then a confirmation dialog popped up on my screeen. I would feel much more comfortable if this service would be deactivated at all.
All you have to do is say NO when asked if they can look at your screen. Or, don't confirm in the dialog. Otherwise you are giving them permission. Simple.
I understand your point. But I am more comfortable if services I usually do not need do not run since it is more difficult to exploit a not running service. Try to harden the MacBook at least a little.