Whats this part for?

[DukeCanBuildit]

It all depends on what material the (Ring) is made of, If Plastic then I agree with a wire loom support, but this style is generally (Round) and not (Flat).
I am picturing it is designed to hold something in place and the closest thing I can find with that design (Tooling) is this

For a Visual Ref: (Very Similar Design)
So, if visualize this and then move down to the actual pics

And with (2) Trucks having the (Retaining Ring) around the same line.

I suspect it is a temporary retaining ring as it rolls down the assembly line, and possibly, this is a physical spot for a (Worker) to transition from the Hardline to the Flex Line, and I suspect that the ring is a temp holder for the Locking (Collar) that is on the bracket.
Or it was a protective cap that covers the end of the line, and it uses that (Slot) for security of the cap - (The Clip is removed, Cap is Removed, Locking Collar installed, Line Connection is made.

It is intended to be trashed, but if not fully removed it stays wrapped around the brake line and comes to rest in the location in the pic.
That is the only logical reason for it to be there, as I cannot see HOW this became magically wrapped around the line (on its own)
It would have to have a (Physical) interaction to spread it apart.

I think you’re on the right track. I wonder if researching that locking collar would show it with a cap and clip? Of course, OE packaging for assembly and retail packaging differ greatly but, it might help solve this puzzle.

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[got3fords]

Found these on McBastard-Carr.