Can you still use a Windows 7 laptop as a second screen on a Windows 10 PC?
No! You can't only use the display of the laptop!
You can answer the question yourself:
Does your laptop have a video input?
For this you need software that provides a virtual second monitor connection on your Windows 10 computer, but which forwards the data sent there over the network, and software that receives it on the Windows 7 computer and on the Screen.
I've never tried it, but Spacedesk claims to be able to do just that: https://spacedesk.net/de
