In Symfony 3.4 how do we log deprections? #47940

It seems the Kernel itself handles silenced deprecation aggregation by adding an error handler (with set_error_handler) during initializeContainer(), which it later writes to ...Deprecations.log if debug mode is enabled. The only use for this file appears to be in LoggerDataCollector, which de-serializes, sanitizes and then presents this data via the Web Developer Toolbar. As far as I can tell, there is absolutely no way to capture these deprecations in any log file.

We need these deprecation notices to help upgrade to Symfony 4, but capturing them is incredibly difficult without logging. To make matters worse, this projects eschews Monolog Bundle in favour of its own logging solution, so even if there was log support for silenced deprecations, it is unlikely it would work out of the box in this project, hence I need to understand how this works (if it works) and what other ways we might capture these deprecation messages.

I should also mention that, on a particular page, two of these deprecations actually do get logged, but that's only two out of a total of 30. For these two specifically, Symfony\Component\Debug\ErrorHandler kicks in and logs the deprecation, but it remains entirely unclear to me why it only does this for 2/30. All 30 of them are triggered by the same @trigger_error($message, \E_USER_DEPRECATED) pattern, so my best guess is just that they are triggered during different parts of the request lifecycle.

Edit: seems like the other 28 errors are captured during container initialization and have nothing to do with the specific page I am looking at; they will be shown on every page until the container is rebuilt.