I recently traded in an iPhone 6+. It can still be useful, but only for some kinds of apps.
For the basic things like making calls, Messages, Notes, etc., it works fine. For activities that require more horsepower (especially games), it is not really useful.
I’ve found many games over the years have had increasing memory requirements (even though they still claim to be compatible with an iPhone 6) and can crash when the app runs out of memory during operation.
Similarly, Safari can get very slow when rendering some web pages. Depending on what you visit and what you can tolerate, this may or may not be acceptable.
Finally, many popular apps now require more recent versions of iOS. For instance, the current version of the IMDB app (12.1.1) requires iOS 14.1 or later. On my iPhone 6, I could not upgrade beyond version 11.0 (if I remember correctly).
There were several occasions where an app developer accidentally entered the wrong minimum iOS version for an update. The result was that the App Store would upgrade the app - which would then fail to launch. I had to complain to the author, then wait for the problem to get fixed, then remove/reinstall the app - at which point the App Store would install the most recent compatible version (the one I had previously installed). It can be quite aggravating when it happens.