is it unsafe if I used Windows 7?
Yes because the security patch of Windows 7 is outdated and ended support by Microsoft 3 years ago and Electrum's latest version no longer supports Windows 7 since the release of version 4.2.0.
That's a good point. There is a similar problem with smartphones, especially android ones, where we have many producers and many OS versions, distributed by phone producer. Sooner or later they stop publishing a new OS updates, then users should think twice if still want to use banking app etc. on not-updated phone.