One reason that accounts for this phenomenon is simply that the speakers have been trained to pronounce them or not to pronounce them, their background (family, region, school) being responsible for this habit forming; this makes it clear that they’ve never worried about whether to pronounce them or not : they just followed a dominant trend ; this is why I personally always pronounce the t in « aout » ; I’ve heard it pronounced so very few times without a final t that I never got familiar with this alternative.

The /plys/ vs /ply/ (more vs no more) distinction is regular in French.

On the other hand, pronouncing the final s in moins is very distinctive.

The tendency is to standardize on the “Parisian TV/Radio accent” but there are still significant regional variations.

See : Ces mots qui ne se prononcent pas de la même façon d’un bout à l’autre de la France. Mathieu Avanzi: