There's so many over the years I've lost count.
Like siri suggestions, spotlight learning your app habits and memories/face detection feature of photos off the top of my head. Some people find those features useful, but not me, so I rather disable them and gain a bit of battery life, no matter how miniscule the gain might be.
Optimized charging feature is not an issue, but mandating the use for significant locations/machine learning for it to work (except 15 series with a hard 80% charge toggle. this needs to go out to all models.) is my problem with it.