Is it permissible to obtain a fictitious profession to match the visa's profession, given that 99% of travelers do this to complete their paperwork, and is what I did by leaving my job with the sponsoring company without the sponsor's knowledge and working elsewhere considered a form of deception?

It is not permissible for the sponsor to allow the sponsored individual to work in a profession other than the one for which they were recruited if the country's regulations do not permit it, except with the approval of the competent authorities. Nor is it permissible for the worker to do so without the sponsor's permission, due to the violation of regulations and the exposure of the sponsor to accountability that would result. However, if the country's regulations explicitly or implicitly allow it, then there is no harm.