I just stumbled upon IronOS, a GPL-licensed open source firmware for “smart” soldering irons.
Always glad to see open source firmware, but I am so old that I didn’t know there are smart soldering irons now??? 
Honest questions: What’s smart about them and why would I need one? And are there open source hardware designs for soldering irons?
The main use case is that they can run off DC voltages, 12v, or 20v, for doing soldering in the field, or portable that can run off a powerbank.
Some can be powered by PD-USB-C, and some are controlled over BT.Pinecil is one of the makes, Miniware the other.
Have yet to buy one.
Brian
These are great! I got a TS-100 when they came out and am now on a Pinecil v2. So much better than anything I’ve previously used (apart from maybe a Metcal). Ability to run off a USB-PD battery bank is really useful, they heat up in 6 seconds, show current temp and they have an accelerometer that cools them down to a lower temp if you forget to turn them off. They work better than my Weller while not requiring a bulky base. The only downside is that recently when I bought a new one and wanted to solder something I instead ended up arguing with a person who wouldn’t accept my pull request to the firmware updater :S
oh and you asked what’s smart about them. I believe you can play tetris on them and they can link to a chrome-based browser over bluetooth for real-time plotting of temperature and power usage but honestly I think the only smart feature most people use is the auto cool-off after a period of unuse. They do have a little “hold button to give an extra boost” feature that I sometimes use when desoldering bigger stuff as well.
Bahahha this is gold!
Seconding tony open source soldering irons! Been using them in the field for a decade now and they rules for when you need to “hack the wild”
I agree with other posters that these are pretty great soldering irons. I got a TS-100 compatible iron several years ago and have never looked back.
https://www.amazon.com/Digital-Soldering-Heating-Electric-Adapter/dp/B09F9SMTKV
If that one ever wears out I’ll be investing in a Pinecil (also mentioned by @DrBrian ) . PINECIL – Smart Mini Portable Soldering Iron (Version 2) - PINE STORE
This is also interesting for off-grid soldering to use portable electric drill (in this case Makita) batteries.
https://mellif-tools.com/en-gb/products/upgrade-adp05-battery-adapter-for-makita-18v-lxt-lithium-ion-battery-with-led-light-and-one-usb-one-type-c-port-cordless-power-source-power-source-only
I’ve soldered straight FROM THE SUN before with a ts-100
Via connecting straight to a panel like this ALLPOWERS 100W Portable Solar Panel Monocrystalline Solar Charger Foldable 331516355170 | eBay
(Note that passing clouds can mess up your soldering ahhaha)
TIL! Thanks for the education everyone. 
Somewhat related is the fully open source Pixel Pump vacuum pick-and-place tool I just saw. Looks useful!