How to reinforce a floor joist with a split knot?

I'm not convinced that this is causing the squeak. The amount of wood that was removed is trivial. I'd investigate further first. Some glue and/or shims or the tightening of a fastener may resolve the issue. Otherwise...

They make steel repair channels for this type of thing, but they're likely not cheap. Your goal is to restore the tension strength of the bottom portion of the joist. Here's what I'd do as a DIY repair, based on decades of home construction and ownership experience:

1. Determine whether the joist has sagged at all. Run a long level or other reliable straightedge across the bottoms of the joists. If that one's low even 1/16" relative to several in either direction, put a jack and a stud under it and give it a gentle lift. Ideally, shoot for 1/16" above level to account for some settling later.
2. Prepare 4-6 foot 2x4 gussets for both sides of the lower portion of the joist. These should be ripped or notched to fit under the cable.
3. Liberally apply heavy duty construction adhesive to the each gusset and fasten them with 2-1/2" gold general purpose screws in pairs every 12" or so. I recommend Torx-drive for easier installation. Keep them at or slightly above flush at the bottom so you don't create a bump in the ceiling.

Be sure that the gussets pull tight with the first few screws. You can't move them with additional screws. If necessary, pilot through the gussets to the size of the screw threads.

Let the assembly dry for 24 hours before removing the temporary post. 48 hours is even better.