Is it okay to leave this old fan circuit capped off like this?

You have the right idea for preserving the circuit, but you missed a few critical hardware components that make it code-compliant. Since you used MC cable, you absolutely need an **MC connector** where the cable enters that junction box. You can't just pull the metal sheath trough a knockout; the sharp edges of the cut armor will eventually vibrate or shift and slice right through your wire insulation, wich creates a major short-circuit risk. Before you tighten that connector down, make sure you slide a plastic **anti-short bushing** (often called a redhead) between the wires and the metal casing. It’s a tiny red plastic guard that protects the conductor jackets from the jagged metal teeth of the armor. Without those two things, you haven't relly made it safe; you've just created a potential ground fault inside a wall. As for the box itself, it needs to remain **accessible**. You cannot bury a junction box behind drywall or cabinetry. If you're closing up the wall, you need to leave the box flush with the surface and use a solid blank cover plate. If someone can't get to it without breaking out a hammer, it's a code violation and a headache for the next guy. Regarding the breaker, you don't necessarily have to disconnect it if the wires are properly capped and contained in a legal box, but it’s a good habit to turn the breaker off and label it as "Spare" or "Abandoned" in the panel directory. It adds an extra layer of safety so that line isn't sitting hot for the next ten years for no reason. Just make sure the box is securely strapped to the stud within 12 inches of where it enters the box so the cable isn't flopping around back there.

You realy want to make sure those caps stay put if that line ever gets bumped. I always swap out basic twist-on nuts for Wago 221 Lever-Nuts when I'm terminating a circuit like this. They lock onto the wire way better than those yellow ones, expecially if you aren't used to twisting them to the right tension.

If you’re dead set on taping them for peace of mind, only use 3M Super 33+ Vinyl Electrical Tape. Most of the cheap store-brand stuff just unravels and turns into a sticky mess when the temprature changes, but the 3M stays stuck forever.

Since you're usin' a metal box, don't just cap that ground wire. You've gotta bond it to the box itself. Grab a green ground screw and loop that copper wire around it in the back of the box. If you don't, the box isn't grounded, and that MC sheath won't trip the breaker if a wire somehow shorts to the side. Also, hope you left enought slack in there so the next guy isn't fightin' for every inch of copper later.

Spot on with the grounding advice. Since you're using a metal box, make sure you also use a metal blank cover plate rather than a plastic one. If you use a metal plate, it'll be bonded to the grounded box trough the mounting screws, wich adds that extra layer of protection if somthing inside ever fails. Also, when you're labeling it, write the circuit number on the inside of that cover plate with a Sharpie. It’s a huge time saver for the next guy trying to figure out which breaker kills the power to that box without having to hunt through the whole panel.

Back in the day, we’d just shove a wad of friction tape on the ends and call it a Tuesday, but don't do that. Since you’re already in the panel labeling things, take a second to acutally move that hot wire off the breaker. I’ve seen too many 'abandoned' lines stay live for twenty years untill some plumber puts a sawzall through the wall and gets a nasty surprise. Cap it in the box like you did, but land that wire on the neutral bar or just tape the end in the panel. A dead wire is a safe wire, no matter how many nuts you twist on it.

Quick update—thanks for the help! I went back and added an MC connector and a green ground screw to bond the wire to the box like you guys suggested. I also swapped the yellow nuts for some Wagos for peace of mind and put on a metal blank cover. Feeling much better about the setup now!