Fender Amp Transformer Rusty Corrosion.

[Sweetfinger]

When it comes to rust and guitar gear, the only time rust is an issue is when it's on a guitar pickup's magnetic pole piece. The rust will eventually eat into the pickup's coil.

The coil wire is insulated. The polepieces could rust entirely away and the coil would be fine.

The rust will get the coil thru sweat and corrosion. It happens all the time. It’s very common.

Sweat can get in the coil and corrode areas where the wire insulation is damaged or act on the stripped ends soldered to lead wires or eyelets, but rust on the polepieces doesn't cause that. The sweat is what corrodes each simultaneously. It's like saying you shouldn't let a banana get too brown because it will make your milk go sour.
I actually haven't seen many pickups wrecked from sweat. It's almost always people mis-handling them, or a fatal defect from manufacture. If I get a dead Fender coil and there aren't any signs of abuse, I re-flow the solder on the eyelets and make sure I scrape the iron tip on the coil wire wraps in the eyelet. About one in three will start working because there wasn't a good solder connection from the factory.

[OffYourFace]

I know what you mean, I’ve saved many old pickups that way myself along with a heat gun but a lot of people in the repair biz will say the rust eventually gets into it, including a handful I know personally. There’s lots of YT videos showing this. Maybe it’s more of a cautionary sign I guess? If you see rust on the pole piece there’s a good chance it’s from sweat and that the coil may be compromised. It’s not a death sentence so to speak. But my original comment meant that’s more of a concern than a rusty transformer.