Anybody can report a site as unsafe. It could be part of the Microsoft/Google squabble;)
As concerns Smartsceen, it has been considered a Microsft spyware by some security specialists, and it may introduce a security breech that can be exploited by hackers, because its encryption algorithm is obsolete and vulnerable. It can be disabled. Or you can use Firefox instead of IE.
I did not "upgrade" my W7 to W10 either, but my grandson did; now, when I send him an application, written and/or compiled by myself, either it is blocked in his Microsoft mail software, and he cannot even extract the attachment, or the app does not work.
I never saw an antivirus blocking a malware in all my computing life since the 80s. It detects old malwares, never the new active ones. And it generally detects at least 50% of freshly compiled software as malware.