We don’t support a "naive" backend for gate application, only for MPO*MPS contraction. Can you just use a cutoff of 0 or a very small one (eps(Float64))?
We didn’t support it in older versions like v0.3.39 either, it was just ignoring the method keyword argument, while now we catch when incorrect keyword arguments are passed and throw an error ([ANN] Change to keyword argument behavior in ITensors.jl).
There isn’t an in-place version, given that the tensors generally have very dynamic sizes it is not straightforward to reuse memory, though that is something we have investigated and hopefully we can do it in a general and automated way at some point.