baobab is available on Ubuntu, and it's GUI app has that capability.
Other options here:
https://serverfault.com/questions/386784/linux-trigger-a-real-time-alarm-on-a-low-disk-space-condition
but if the C code listed in the answer there is the approach, might as well do something similar in the box code.
Similar here: https://www.linux.com/training-tutorials/get-disk-space-email-alerts-your-linux-servers/
Having it be a part of the standard system health check would be 👍