Have a look here
Particle Firmware Updates Thread
Help creating STRING variables
It never was a good idea to use a String object for a Particle.variable() since the actual stored string might get relocated causing the “link” between the string and the Particle.variable() to break.
Before you weren’t warned about the risk, but it would have happened. Now you get told beforehand that this may break your code and hence will not build.
Sorry I must be reading over it, where is the spelling mistake?