There are private companies that claim backport security updates, although I don't know how effective they are. Their customers are primarily government and defense.
On what basis? Depending on your crankiness level you could argue that "backdoors" far predate that[1].
Despite every major news outlet reporting this back in 2019, this is actually fake news. People saw that the taskbar appeared blue in a low-res photo, and concluded that it must be Windows XP. Very sloppy journalism.
If you look at the full-resolution version of that 2019 photo, it's clear that it's Windows 7 (or maybe 8 with a Start menu mod): http://static.kremlin.ru/media/events/photos/big2x/W2kaDAtDz...
Although it is very possible that more recent proprietary OSes contain spyware, Putin showing himself using XP is almost certainly propaganda to fuel distrust towards Microsoft and western governments/corporations in general. They have top notch FOSS developers in Russia, and I'm sure he can obtain a 100% spyware free Linux/BSD/whatever PC, unless he doesn't trust the hardware itself which can be bugged just like OSes and software (firmware, binary blobs etc.) so that he uses very old iron that most certainly don't contain malware but can run only ancient OSes. If that's the case, then it could make sense.
AMD's licensed zen1 chips that China makes, whose name excapes me at the moment, have the AES instructions and PSP removed. They're also banned from being imported by the state dept, so, that's either because they're also backdoored by china somehow, or, they aren't backdoor-able, maybe both.
Suffice to say however, modern computers (and smartphones) are chock full of potentially spooky SoC's and someone like ol'vlad might just be paranoid enough to not trust them.
That said, I have read many times that Putin does not use any computer or the Internet.
(For those unfamiliar with it, the ME is an inaccessible second CPU running Minix and is network-aware.)
I wouldn't be surprised no. He's got flunkies to do everything for him. And he does seem pretty out of touch with reality, which might be because they tell him only what he wants to hear. For example consider the war on Ukraine, it seems he really expected to overrun them in a day or two.
Hardly. He probably has more important matters to think about. That would be a world record in propaganda with the least impact.
There's no reason for him or anybody else to use inferior FOSS software when an old OS does just fine.