[julie1]

And this ? https://zh.wikipedia.org/wiki/%E6%91%84%E5%8A%A8%E7%90%86%E8... (pt https://pt.wikipedia.org/wiki/Teoria_de_perturba%C3%A7%C3%B5...) (fr https://fr.wikipedia.org/wiki/Th%C3%A9orie_des_perturbations)

[ovis]

Yes, perturbation methods are still taught and are still recognized as important. The first math course I took during my graduate degree (Intro analytic methods) covered it, for example.