What will happen if I’ll use Windows 10 with Office and Photoshop in VirtualBox inside Linux completely offline? Can my date leak?
Windows 10 is primarily a release of Windows that works best when online. That will include the telemetry that Windows 10 collects, to send off to Microsoft. In this case, nothing can be sent without an online connection. That doesn’t mean Windows 10 cannot store information and send it to Microsoft when a connection is established. Since this is a guest OS running in VirtualBox, there shouldn’t be a problem.
Buy why do people use open source alternatives instead of this method?
Well there is WINE for people who absolutely need to run any software for Windows. Libreoffice provides an extensive amount of options. There is also the possibility that the machine running, may not have hardware that can run a virtual machine. People choose free software, because the software doesn’t contain code that will gather information. These are the many reasons why people will choose free software, instead of running a guest OS.
Did you mean : that doesn’t mean Windows 10 cannot store information and send it to Microsoft when a connection is established ?
Also, don’t some of those programs require Internet, at some point, to check with the publisher that they are legitimate, activated, etc.?