If you don't need 5G that's not a problem. Many are happy with LTE still.
It supports VoLTE (heck, so does the much-older Galaxy S5).
Don't let the fear mongering about security bother you. If I were still interested in smartphones, my S5 would still be plenty good. Just don't click strange links in emails, or visit porn sites, or install sketchy APKs and you'll be fine. There's far more telemetry and background data going on in a modern phone so you're actually more at risk with a modern one.
The only downsides of the S6 are battery life (the phone was the first to ditch the removable battery and any left over models are long past the useful life and it wasn't that great to begin with), zero water resistance, and zero expandable storage. But it has a headphone jack and IR blaster still! So enjoy the ability to mess with bar TVs!
App support will be fine if your uses are not expecting much. Don't expect most games to still work, and don't expect many banking/shopping apps to work (Walmart and Kroger as well as Little Caesar's work fine and the Green Dot and Amex Serve APKs still support 8.0)
Don't update any apps. Leave them at the factory version. otherwise the phone will lag. Turn off Play Store updates or better yet don't sign into a Google account and use an app store like F-Droid or Aptoide.
If you just need it to text, call, play music, and browse the web, it's perfect for that. Old smartphones are great ways to digital detox, or prepare yourself to downgrade to a flip phone later.
Just to help with security, I'd keep mobile data off, and use secured wifi when possible. Many top-tier Antivirus apps like Avast or MalwareBytes still support the S6 so that should help as well.