As BMW chooses not to provide battery information after each service, I suspect that this type of concern will increasingly emerge as batteries get closer to their end of warranty period. I confess that I have always gone on range. When I sold my 2015 i3Rex in March this year, I hadn't seen any range degradation but I accept that is not to say that battery was still charging to 100% of what it was when I first purchased the car.
I am pretty sure that if I took my i3 in for a battery check in the UK, my dealer would want me to agree to pay for the diagnostic test and labour. Clearly, if the test showed that there was an issue covered by the battery warranty then there is a discussion to be had.