The OnePlus 6 and 6T are pretty well supported in postmarketOS, below is a list of features that DO NOT work:
Calls / Texts / Mobile Data
Whilst currently non-functional, upcoming MRs to ModemManager will enable support for at least SMS and mobile data.
The cameras are missing sensor drivers, these are hard to find and to get working, however the sensor end of the ISP is gaining support thanks to work by Linaro.
The headphone jack has support which will be coming to postmarketOS fairly soon, hopefully the earpiece will too. The speaker will require a new mainline driver.
The sensors live on the SLPI (Sensor Low Power Island). Whilst it just connects to regular i2c/spi sensors, it doesn't expose the controller to Linux, a new framework will be needed to talk to it.
All other features are either partially or fully supported. The OnePlus 6 uses the
linux-postmarketos-qcom-sdm845 kernel package which is a close to mainline fork based on the latest upstream release. I think I've put in a fairly significant amount of work into porting, upstreaming and maintaining the device, I think it's a pretty good fit for community now.
Edit 1: With the latest commits, OSK-SDL is now functional with hw acceleration. I've also bumped the device package versions to 1 as that seems more in keeping with other device packages (is there a recommended approach for this?).
Edit 2: I've reworked the history to make more sense and applied Minecrells suggestion for the initramfs-firmware subpackage. I've just remembered a slightly frustrating bug that I'm yet to fix - that being the RTC on the device being somewhat broken, it's not possible to write to it, only to read, and the value read represents the time since the device was first powered on. This means that until you've connected to wifi and ntpd has refreshed (which can take up to 15 minutes!) the date will be wrong and browsing the internet will result in SSL failures. Any ideas on how to approach this?