Answer
(Fatwa: 1286/809/D=1432)
It is proved from hadith to supplicate after fardh salah. Its virtues too have been mentioned in hadith, but it is not part of salah. If someone stands without supplicating or winds up his dua earlier it is allowable. However, one should not do long dua after those salah that are followed by sunnah rather it should be too short. Also, it is better to supplicate silently, sometime one or two words should be loud by which the purpose is to draw the attention of people towards it or teaching them, then there is no harm. Hence, it is against sunnah to do long dua after salah as well as to ban loud dua.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband