In message , at 03:03:02 on Tue, 16 Oct 2018,
John Levine remarked:

It's also a reminder that TfL is a bit naughty to omit both this line and
Thameslink from the Tube map. It's not helpful to passengers, who don't
care if a line is run by LU, LO or GTR.

It's on the rail and tube services map which is the useful one these
days.

Showing it on tube maps would tend to lead people to expect a more
frequent service than it offers. For an inner zone line, anyway.
--
Roland Perry