I you look at that data then you can see that the oldest version with any significant usage is v8.1. But even that is quite low, and you might do quite well in not supporting anything older than v9.0.
But most of that can be done with polyfills and transpilers, which the android gradle plugin does automatically nowadays.
How do you say this without any knowledge whatsoever about the type of app they work on, or their target audience? v6.0 alone is almost 2% of users based on that chart. In fact if you include everything before v8.1, you seem to get something like 8% of users. What if their app, say, provides the poorest people access to public transportation? Would you really just drop 8% of people on the floor in any scenario?
Is it a pain to support old SDKs? Yep. But that doesn't make it inherently a waste of time to do so.