My experience is very similar to what @milancurcic describes. Most of my Fortran programming at the moment is on old(ish) projects which already have their own home-baked versions of stdlib utilities. I haven’t had a chance to either update these to use stdlib (definitely on the radar for some of the bigger ones) or start new ones using stdlib.
I also find myself writing more Python than Fortran, especially when performance isn’t critical, which has historically been because of easy access to utilities like those provided with stdlib, package management and ease of use in Jupyter Notebooks. All of these are things being addressed by Fortran Lang and so I see myself having fewer and fewer reasons to turn to Python in the future. I suspect a lot of folk and projects are in the same situation.
So, give it a few years and I reckon stdlib will be ubiquitous!