Its security depends on you, the item between the chair and the keyboard.
I've been using AV-less systems for I don't even remember how long now. Over 5 years at least. And during that time I've not gotten anything resembling a virus.
There were some stuff that I profylactically checked for and was discovered as malware but even that was a false-positive IIRC.
Me personally, will be staying on Win7 for as long as possible. It's MS last decent OS and seemingly the last decent kernel aswell[6.1.7601, Vista.1 bois
], Win 8 - 8.1 - 10 introduced certain features that some programs and my mainboard seemingly don't like. It's minor in the bug-sense but extremely annoying from the users perspective. (In some programs the cursor randomly resets its position time to time etc.)
Plus the damn things are spying on you. And there's a simple rule of thumb here. If the product is for free, then it aint the product, you are.
That OS is highly the most backdoored OS to date. Pretty sure of that. Windows 7 might still have some as well, but considering its age and the amount of 3rd party work done on investigating it, most of the stuff seems to have been ironed out.
Just make sure you don't really use any proprietary applications on them if security and privacy is involved. A good example being bitlocker. Never store anything you dearly care about in the cloud without it being securely encrypted with open-source and popular products.
My 2 cents.