Just go ahead and ask them. I mean I don't know anything about your job, but usually nowadays there is little reason to stick to Windows with Office 365 and stuff.

Most of the time it just never got challenged because no one cared enough. So it's not like they "insist on Windows". It's "they didn't bother yet".

Of course you'll likely be faced with the myth of maintenance. Counter that with security (homogeneous environment is very good against malware and such) and missing license costs to convince the C*O. Done.