In 4.2 if you specify a log file that can't be created an error message is written to stderr before mongod exits:

> mongod --dbpath /tmp --logpath /nonexistent/directory
2020-07-05T09:51:55.287-0400 F  CONTROL  [main] Failed global initialization: FileNotOpen: Failed to open "/nonexistent/directory"

In 4.4 mongod just silently exits, giving the user no clue what the problem is.