mastodon/config/initializers/rack_attack_logging.rb

ActiveSupport::Notifications.subscribe('rack.attack') do |_name, _start, _finish, _request_id, req|
  next unless [:throttle, :blacklist].include? req.env['rack.attack.match_type']
  Rails.logger.info("Rate limit hit (#{req.env['rack.attack.match_type']}): #{req.ip} #{req.request_method} #{req.fullpath}")
end