I just noticed your signature, I recently was curious and did a comparison of some brands to see who was still supporting their older handsets and, sure most here won't like it, Apple seems to be the best after purchase support. I mean the iPhone 5s is slated to get the 12.4.2 update being rolled out and that phone was release back in 2013

In comparison, the Samsung Note 3 was released that year and it seems the last
official release was to update to Android 5.1.1
How's OnePlus doing? Well not terrible, but I believe the 3/3T series won't be receiving any major update past Android 9 and are now only slated for security patches. Those phones are the 2016 offerings from OnePlus. So technically it'll be interesting if OnePlus even puts out any security patches (the 3T was launched in November of 2016 so is the last security patch to include November 2019?)
And for anyone interested (this info is available ALL over the web/internet) here's OnePlus device offerings over the years:
One 6/2014
2 8/2015
X 11/2015
3 6/2016
3T 11/2016
5 6/2017
5T 11/2017
6 5/2018
6T 11/2018
7/7 Pro 05/2019
7T 10/2019
Click to expand...