Navigation lights are a theoretical necessity, though you'll need a few more to make up for the loss of up and down. Port, Starboard, Top, Bottom, Bow, Stern. You can now tell at a glance the orientation of any ship. How you light ships that rotate for gravity is not something I'll get into now.
Turn signals are for close maneuvering, it's to allow someone behind to react to something you're going to do with appropriate notice. Nice, simple concept. Except it doesn't work.
Let's start with a quote from one of the greats, whose work I'm sure you're all familiar with.
Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.
Why is this relevant? Dispersal of light, ships are not going to be close enough for navigation lights to be seen.
We talk of ships passing in the night
Ships that pass in the night, and speak each other in passing,
Only a signal shown and a distant voice in the darkness;
So on the ocean of life we pass and speak one another,
Only a look and a voice, then darkness again and a silence.
Ships passing in space? They might hear the voice in the dark, but the signal shown will be lost in the night. At best a twinkle in the darkness in the vast spaces between the stars, lots of stars, drowning any little navigation lights you might have.
Your turn signal, it lets ships around you know what you're about to do, except it's too late. By the time they're in range to see the turn signal, especially STL only, it's far too late to do anything about it.
There's a saying with hovercraft, that if you can see something in front of you, you're going to hit it.
What this means at interplanetary or even orbital speeds is that if you or anyone near you is operating on manual, you're going to die, or they are, or everyone is, most likely everyone. If you're in a situation where you can both see and need to respond to a turn signal, write your will, quickly.