not a big concern, not life threatening
my view: Code sets standards to account for hazards and danger. Sometimes Code has to set minimums for things that are not all that serious after all. Perhaps in some future decade, century or millenium, there will be two different documents covering the same as (or more than) what Code now covers: the first one covering hazard / danger / safety concerns, and the second one covering permissible calculations allowing one to build options for non-crucial work-arounds.
That being said, let us now look briefly at the diameter of the pipe that takes shower water away from the shower floor and carries it downstream to the stack. Did you know that Code used to set that diameter at 1-1/2" (1.5") ? That minimum was raised to 2" at some point and this has now become the "norm" everywhere in the US and Canada and perhaps in a few other places that I don't know.
To go back to first principles analyzing
why 2" now and why 1.5" was once deemed acceptable, can be a short or a long discussion depending on how much depth you want to dig into the issues.
Carrying capacity of pipe is the short answer.
If your 1.5" pipe alreay in place has the right slope (and not too much slope, i.e. steeper) and if the distance is short enough before the first Vent, then its internal capacity to exchange air and water will be as good as the original (1.5" shower drain) Code called for, and it'll work well BUT it is very likely that no-one will want to leave any trace of anything hinting at a clear recommendation that you do anything regarding this well-installed 1.5" drain pipe.
Not a plumber, I looked into this same subject for many years. I renovated my own two bathrooms. In one bathroom a Master Plumber who I hired installed a 1.5" pipe and P trap (instead of a 2" which I had specified and which he had accepted to do); he claimed that for the short distance involved it would work fine, and that he didn't find the necessary space to put in 2" fittings and pipe.
Do not ask me now whether "Xx distance is short enough" so that you can safely .... since three variables, 1. slope, 2. venting and 3. distance, all combine to make this situation a bit too complex to talk about unless A. you are truly competent and B. the person responding is truly competent. I think a Master Plumber may be professionally accredited and allowed to make some judgement calls based on serious measurements and knowledge of the interrelated constraints, as well as a licensed Contractor in some places, as well as a homeowner in most places.
AFAIK.
d-v-d