Previously we had the functionality to detect and raise warnings when undefined classes/functions were used in libphutil modules. This functionality was broken in D9864. Previously, $symbols was the raw output of phutil_symbols.php. After D9864, it's the processed output of PhutilLibraryMapBuilder, which no longer has the need key.
This issue was identified in T1806 (see T1806#11 and T1806#13).