Rebuild the fallback list in late 2019 or early 2020
Late in 2019, or early in 2020, we should rebuild the fallback list again.
We usually do this when 25% or more go down. (When 25% of fallbacks are down, a warning is logged daily in #tor-bots on IRC, and via email to the fallback maintainers.)
Here are the instructions for running a rebuild: https://trac.torproject.org/projects/tor/wiki/doc/UpdatingFallbackDirectoryMirrors
See the child tickets for each step.