I currently use a pretty old version of unity (2017.4.0f1) and an android from 2017 too. But I’ve been reluctant to update because I assume the more recent updates and the more advanced/heavier unity gets, the worse the performance will be on android, so I just stuck to 2017. Is it true that the more recent the unity version is the worse the performance will get on android or am I thinking the wrong thing?
It all boils down to how YOU optimize and set up your game for optimal performance. That said, 2017 is perfectly fine to use if it meets your needs and 2019 will introduce problems but only if you use new high-end features not intended for mobile!
Make sure to Google and read the many articles out there on this topic:
Forum threads like this one:
And of course the manual: