make-variable-buffer-local should not be called on a variable where it is used, only where it is defined (see its docstring). When the variable is unbound at the time make-variable-buffer-local is called, the toplevel default value gets set to nil, which causes the variable to remain nil in all buffers once company.el (which contains the defcustom for company-backends) is loaded.
Fix #16320
make-variable-buffer-localshould not be called on a variable where it is used, only where it is defined (see its docstring). When the variable is unbound at the timemake-variable-buffer-localis called, the toplevel default value gets set tonil, which causes the variable to remain nil in all buffers oncecompany.el(which contains the defcustom forcompany-backends) is loaded.