Re: "support" is a sales "feature"
I've still got a OnePlus 3, that came out in June 2016, so isn't far off 5 years old now.
It got updated all the way to Android 9 in May 2019, which improved battery life compared to the earlier OS versions.
Depending on usage, (I mostly use messaging, bit of news, and make perhaps one phone call every two days, although this typically lasts up to an hour each time), the phone will typically last between 2 and 3 days between charges.
The only time it's an issue is if I've had heavy use for some reason, such as taking lots of pictures, where it might not last the day, but if I know I'll be doing that, I take my booster with me.
No idea if good battery life is due to how I charge, or the environment it's in etc. I'm in the UK, so no excessive heat or cold, and I typically run the phone down to 15% (when the warning comes on) before charging, and then unplug as soon as it's ~95% which is when it's charging at only a trickle anyway. (Using the original charger and cable from OnePlus).