I vote no on Armor All too
I used it on one of my new cars religiously back in the late 80's to try to preserve the interior snd exterior rubber and vinyl plastic exterior trim on it. Instead, they all ended up degrading quicker. Greying out and cracking just after only four/five years. I also got the stuff on the hatchback pneumatic spring piston rods somehow (most likely from over-zealous use of the stuff on the interior plastic trim), and the seals on both of them went bad and the hatch wouldn't stay up when it was opened. Never used the stuff again after I heard from other people that Armor All was bad for plastic and rubber. Sure, it makes stuff look good right after you apply it, but it definitely shortens the life of the materials in the long run. This had been duscussed in many car forums through the years and many had similar experience with Armorall.
All my other cars since had never been treated with Armor All and their trim held up better even for a lot more years than the car I had problems with using Armor All.

Penetrol works wonders on plastic and fiberglass, maybe on Delrin too?

Armor-all has silicone. Silicone displaces the original plasticizers in many plastics. So, once you go down that road you are married. I do not like it on vinyl coated convertible top fabric as eventually if too much too often is applied the backing fabric releases from the vinyl layer. I did have good luck on a '79 new car on the dash, but I was periodic, like every 3-5 months, no cracks after 11 years, but I knew I was "married" to it. Guy who bought it could not believe the dash was intact. Be careful on new cars, there is a new resin of choice, very unhappy with armor-all.
The 303 brand protectant... the MSDS sheet tells Nothing of use. I shy away from that as I want to know the chemistry.
Delrin is a brand name of nylon, nylon is hydroscopic, it will absorb water up to 3% of its mass and swell. That was a big headache for Toyota a number of years ago on Camry sticky accelerator assemblies.
Mineral oil such as "baby oil" will probably do as good a job as many elixirs, there are a few "back to black" products in the car care realm, some use a dye, a reasonable chance some of that black dye will penetrate into the plastic.
The grey is from oxidation and UV degradation, choose your dose, it has been decades.
The constant tension the plastic front derailleur bodies are under is not a well designed use of the plastic, yes delrin is self lubricating to a point but the expected use was under compression, not tension.
I have lost a few front derailleur bodies with the assembled bike just sitting in the bike room, yes, I did not know the bike's entire history but I don't trust that application much now at all.

While I have no used on bike parts specifically, I have used Frog Lube on other plastic items to bring them back. Actually designed for use in gun cleaning, but works great on lots of stuff and they actually make a chain lube as well now. But I use the stuff designed for guns on plastic.