I managed to extract these Mastodon federation API endpoint (filename, [endpoints]) tuples from the code. No idea if this is complete or what.
accounts_controller.rb:
show
follower_accounts_controller.rb:
index
following_accounts_controller.rb:
index
statuses_controller.rb:
activity
show
tags_controller.rb:
show
activitypub/collections_controller.rb:
show
activitypub/inboxes_controller.rb:
create
activitypub/outboxes_controller.rb:
show