Why aren't charger IC's smart enough by now to avoid this? (eg. let it drain a ways before topping off again).

Does iPhone’s optimized battery charging help?

Not sure. I actually had an iphone with battery so swollen, the screen got popped out of the frame by itself. Maybe newer iphones is better at this stuff, but why risk your house burning down when you can take some precaution regardless the type of phone you use.

I have a 5 year old Sony Xperia ZX which has had "battery care" enabled (possibly by default) since I bought it.

It charges the battery to something like 85% until shortly before my alarm is set, or the expected time I'll unplug it if no alarm is set. There isn't a way to manually control this, beyond changing the alarm time.

However, the battery is still going strong after 5 years.

(I wish they'd make a decent compact successor. The latest "Compact" Xperia phone seems to be 2.5cm taller! https://www.gsmarena.com/sony_xperia_5_iv-11838.php 15.6cm vs 13cm)

Just plug the phone charger in a power outlet timer that turns on the power just 2 hours per day. That would simulate normal daily phone charging.

This is a great idea. Can even automate it using home assistant for precise control if you're using android. Home assistant's android client reports battery life to your home assistant instance, so you can write an automation that switched on a smart switch connected to charger when the phone's battery <50% and turn off when it's >95% (or other values optimized for battery longevity.

Don't modern battery charge control chips take care of this automatically?

I mean, many laptops spend 99% of their time on plugged in/docked, and that doesn't cause any battery problems.

Wait, what? Aren't batteries and chargers smart nowadays? I thought they automatically care about battery health no matter what the user does (apart from deep discharging of course)!

Modern phones have an option to stop charging at 85%, which mitigates some of the issue as well as extending the battery lifetime. The main thing is the battery needs to go through charge cycles, sitting is death.