Not exactly sure what your requirements are but vanilla forums have a clean look, mobile friendly, and have pretty good customization.
(Not sure if the latest version supports digest emails though)
"The master branch is considered a stable branch capable of being released at any time. Reviewed, stable changes land against master via pull-request."
Last update was jul 13.
It's pretty stable and has been around for a while, so i wouldn't expect a flurry of constant changes (durability and stability are exactly what i look for when self hosting)