Practically speaking, this takanah isn’t kept as much as it should be – I have many times gone in to Stenhouse/Dansky’s during the Yeshiva’s bein hasedorim (boys’ times) and found the place teeming with American girls, perfume and all. And yes, most of these girls are American (they chatter so loudly it’s impossible to not overhear their accents).
And when the Mashgiach of Gateshead Yeshivah, HRH”G Rav Mordechai Yosef Karnowsky SHLIT”A walks in when there are girls there in boys’ time, he ensures that they leave promptly. This is quite a clear demonstration that Da’as Torah is that this takanah is ongoing and anyone daring to breach it is being poretz geder.
For further reference, see Michtav Me’eliyohu (Rav Dessler ZT”L) vol. 1 page 75 where he quite clearly says that a peson who goes contrary to Da’as Torah is essentially eating away at the foundations upon which Yiddishkeit is built – however blindingly obviously wrong, senseless and counterproductive the opinion of Da’as Torah seems to him.