My modernest operating system is a current distro of Ubuntu.

Still in love with my Mac OS 10.6 (Snow Leopard) tax machine (offline).

I keep another machine of the same era (Intel Core2Duo) online with Win7Pro, for official paperwork/logins. Doesn't seem to be hacked / compromised, yet (what people usually say).

Also rocking modern Apple Silicon (M2Pro/3/4) which is impressive equipment, particularly considering their miniscule power usage. The current 15" MacBookAir will stream video for the majority of a day on a single charge, and from CostCo can be occassionally purchased for $849 (which includes an additional year of warranty).