I'd like to remove some deprecated code from CiviCRM core, but I'm not sure if it might be used by other extensions in the CiviCRM universe. Is there a way to search the code of all known extensions?
You can use build kit to generate a repo including the civi "universe" https://docs.civicrm.org/dev/en/latest/tools/universe/
civibuild create universe
mkdir ~/src/universe fetch-universe ~/src/universe
could be worth checking extension usage to see if there are any widely used ones that don't make the cut to get included.