Go check in Aliexpress: there are tons of non-smart phones, especially the stuff marked as "senior phone", and they're pretty cheap too (like $15 for a mobile phone that just does calls and SMS).

If you want the stuff that's not glitzy and heavy on marketing you need to get it from where the factories are, not were the brands are - basic mobile phone tech is a thoroughly solved problem and highly integrated nowadays and well within range for even smallish electronics manufacturers to design themselves.

Also check HMD, the Finnish mobile maker who bought Nokia's mobile business, who also have several non-smart models (including old Nokia models).

Edit: No idea if any are flip-phones though. Here's an example flip phone

I had a Sharp SH-03L for a while, it's a business version of one of their flip phones that didn't even have a camera.

The OS was actually android 8.0 but really stripped down to basically only do the whatever apps a flip phone has.

I was able to sideload apks through ADB, but ironically, I actually wanted the google stuff to work since a lot of the apps required it to log in and other things.

The thing was pretty cheap though, paid like $15 for it

Ha yeah. I would absolutely confirm there is no data connection

You guys have to print out tax forms?