Speaking from experience after working throughout the day to revert my 2012 Macbook Air from Catalina to El Capitan... I suppose this all began when I upgraded to Mojave a while back thinking it was the latest and greatest at the time. Ends up it wasn't, but apparently auto updates had gotten enabled, and I'm usually hesitant and heavily delayed in upgrading due to suspicion of higher resource requirement and generally added system slowness with each subsequent update. So to my surprise when I saw my mac rebooting automatically for another upgrade, I was ****** but curious as well. Long story short, as soon as I got to Catalina, my battery lasted for about 30 minutes full charge (once disconnected) with a new 'Service Recommended' status. There was no behavioral changes or indicators that would suggest the battery was degrading. It was perfect condition for the most part. Obviously it's why I still have a 2012. I was thinking "oh crap, no more portable mba...", and then decided forget this, I'm reverting to a more stable and less resource intensive OS (El Capitan). Ended up finishing the wipe and re-install successfully and I can already tell the difference with performance. I'm also going to casually throw out there that my battery now reports 'NORMAL' condition and the battery life has increased ten-fold back to its original state. It is quite the joke honestly as I now have proof my battery is in perfectly working condition along with the MBA itself. It is 100% the OS "upgrades" to blame that are breaking these devices over time. Seems to be a bad company practice, but hey what can we do besides revert to a less demanding OS. I highly recommend it.