Powder insulation is often asbestos which was used many years ago to insulate things including ductwork and stove pipes etc.
Asbestos is dangerous if breathed in through the dust particles so the asbestos should not be touched or disturbed.
Some wires also used a powder insulation inside the wires which is sometimes also magnesium oxide.
For example a cable that uses magnesium oxide are MICC cables.
The magnesium oxide protects the MICC cables copper conductors.
MICC cables are also called pyro cables and are made of copper conductors inside a copper sheath and are insulated with the magnesium oxide powder.
In other low voltage cables and and other kinds of cords the white powder is often just talcum powder, which is used in some low current and low voltage cables.
The talcum powder is added the inside of some cords to make the cord for flexible and prevent the internal wires from becoming stuck to the outer rubber casing.
When the cable is used over a long period of time, the cables outer rubber becomes sticky and can be broken down.