oxidizing copper is toxic tho
While it is true that copper oxides are somewhat toxic, Copper is a "self-protecting" metal. When the oxide forms, it forms a hard, dense layer that actually protects the wire from further oxidation. Essentially, only so much will form, then formation stops until the wire is flexed to crack the existing layer of oxides. Copper oxides are not very soluble in water, the amount leaching into soils is minimal, over the 2 to 10 years interval between repotting that most in the hobby schedule, so "copper poisoning" of our bonsai trees is simply a phenomena that has NOT been observed.
Do not worry about "copper poisoning" from copper wire. It is simply something that does not occur under the conditions normally encountered in bonsai.
Copper and iron are necessary plant nutrients. Both are needed in small amounts. Both copper wire, and rusting steel wire, and for that matter rusting galvanized wire are not likely to be an issue. Rusting wire is unsightly if exposed.
Galvanized wire contains zinc. Small amounts of zinc are also a required plant nutrient. There should be no serious issue with rusting galvanized wire as far as toxicity goes.
Rusting or corroding wire can pose a problem. We wire the trees into their pots to prevent jostling from breaking roots. Tall trees in shallow pots often are top heavy, and will flop out of their pots even if the roots have filled the pot. Rusted or corroded wire, when it breaks ceases to stabilize the tree in the pot. The next big wind, frolicking squirrel, stray cat or clumsy owner can knock the tree out of the pot. The wire is not just to hold the tree in place until the roots are established, but rather to hold the tree in place through the entire time it is in the pot. Even established trees, especially tall trees in shallow pots need to be wired into their pots. So as a general rule, corroding or rusting wire can lead to a tree flopping out of its pot, usually at the worst time of year for a repot.