MIG wrote:

The N89 is one of the oddest routes, because while many routes have to
do a direct trip from Trafalgar Square to where they join their normal
route, eg the N47 and N21 at London Bridge, the N89 has to find an
route all the way to Lewisham, and it's nothing like the route of any
day bus.

It's not a particularly unreasonable route from Trafalgar Square to
Lewisham, though. The N41 route from Archway to Trafalgar Square is odder.