Aside from the use of B1 in tissue culture media, where it seems to be useful if not important to the process (I know little about TC), the current reasoning behind the continued acceptance of the myth of B1 being a root stimulant or anti-transplant-shock treatment is due to studies from the 1930s-40s using B1
and other known hormones in the treatment mixtures. Yes, B1 was applied to the plants. Yes, the treatment group(s) had some degree of improved rooting/root growth, but the root stimulating effects have since been credited to the hormones in the mixes. B1 was just along for the ride and received more credit than was due.
Studies with only B1 or B1+other nonhormonal nutrient ingredients (example
here) have reliably shown no effect on root growth of seedlings or cuttings.
Is the use of B1 going to hurt plants? Probably not.