The endpoint we could not deprecate because nobody knew who used it

An endpoint from 2019 with no documentation, no named consumer and four hundred requests a day.

// logging what we actually needed, for a fortnight
Log::channel('usage')->info('legacy_endpoint', [
    'api_key_id' => $request->user()?->apiKeyId,
    'user_agent' => $request->userAgent(),
    'ip'         => $request->ip(),
]);

// the answer: one key, one user agent, 400 requests a
// day, all at 03:00 — a cron job at an integrator we
// had a support contact for.

Two weeks of logging turned an unanswerable question into one email, and the integrator moved within a month. The general lesson is that “we cannot deprecate it because we do not know who uses it” is a statement about instrumentation rather than about the endpoint, and the instrumentation is four lines.