The root/ROM community in Android may not be as big as it once was, but there are still hardcore enthusiasts out there who refuse to buy any phone unless its software is modifiable. For those of you, there's good news - SunShine now supports S-OFF for HTC's latest flagship, the U11.
— Jon Sawyer (@jcase) August 26, 2017
If you don't know what S-OFF is, it's essentially an unlocking of HTC phones' security that enables users to modify system partitions (and have them survive reboots) and flash unsigned firmware zip files (see this XDA forums post for a more detailed explanation). From here, you can flash custom firmware to your heart's content. It's also worth noting that the U11 is the first device SunShine supports that only has temporary S-OFF, meaning that you'll have to rerun SunShine every time you reboot to recovery or the system. (Thanks, Kisakuku.)
You can head to theroot.ninja (yes, it's a cool URL) if you're interested. SunShine does cost $25 per device, but it's well worth the money if you'd like to have lower-level access to your device. Plus, developers have to eat too, don't they?