I grepped through scripts/ again for any references to this module with:
grep -R '^module Thread' ./*One hit. In scripts/base/init-bare.bro. This file is hard coded (linking to lines in Github generally isn't immutable, so search for 'add_input_file("base/init-bare.bro");') to load as long as Bro isn't run in "bare" mode (a mode where Bro loads a minimal set of scripts). So I opened this file and found a very small amount of Bro script belonging to that module's namespace. In the module's export declartion was a single re-definable constant called "heartbeat_interval". It was set to 1.0 seconds. There were also some comments stating that changing this value will likely break some things.
Interesting. So I searched through scripts/ again, this time looking for references to "heartbeat_interval". I found zero use of the value. So why was it defined at every Bro invocation?
At first I thought it must be dead code. Perhaps the module was left over from an unfinished feature or perhaps it was from a feature that was available in an older version but longer supported. Large C++ projects are often notorious for having dead code. But this is the Bro team; I should have known better.
I went to Github again and looked through the blames around that line in inti-bare. The definition of heartbeat_interval was done in a merge named "topic/robin/input-thread-merge". I was still confused. So I grepped through src/ to try to find where this value was used. This time I found a few different .cc files.