TLDR: If you use $wgDebugLogGroups['profileoutput'] in LocalSettings.php as
a way to collect performance profiles from Tideways/XHProf, this will no
longer receive messages in MediaWiki 1.35 and later.
I've been auditing the Profiler component in MediaWiki and deprecating or
removing various legacy features that either don't fit the current model
very well and/or that seem to be broken, unused or undocumented.  The
objective here is to keep offering the same functionalities for developers,
but in a way that's easier to use, with fewer moving parts, and lower
maintenance costs for us.
Today I'm writing about the "profileoutput" debug log channel. As part of
the PSR-3/Monolog refactor many years ago, this feature was grandfathered
into the LegacyLogger.
The $wgProfiler variable controls which collector and output are used at
run-time. For example, you can collect the call graph with XHProf/Tideways,
and then output it to an HTML comment, or dump to a file on disk, etc. Any
number of outputs can be implemented and enabled.
The "profileoutput" debug log channel was effectively another way of
achieving the same thing, by assigning $wgDebugLogGroups['profileoutput']
to a file path.
If you're currently using this, see
for how to make this work via