It’s very likely that the custom of upsheirin was relatively unknown in Europe before the war. Indeed, they probably didn’t do any celebrating on Lag Ba’Omer, aside from omitting tachanun. Nevertheless, we see that the Chasam Sofer’s objections to this celebration have not been accepted and nowadays nearly all Chassidim today do celebrate it in one form or another. As recently as 2 weeks ago, Rav Chaim Kanievsky told questioners not to denigrate the custom of celebrating at Meron on Lag Ba’Omer. It was for him (like so many other questions), primarily a question of bitul Torah. The Arizal’s custom on this matter might not have been so widespread in Europe, even while his nusach in davening was. So, it’s understandable if some rabbonim might be hesitant to accept it. But calling it a “minhag shtus” is going way too far, just as no one would CV”S dare call Nusach Ari a shtus.
