Assalaamu Alaykum Shaykh, I wanted to ask in Ramadhan after Taraweeh salaat is finished, why is witr salaat prayed in Jamaat and after Ramadhan why is Witr Salaat not prayed in Jamaat anymore after Isha Salaat.
That’s the sunnah.
According to a hadith of Jabir (ra) the Prophet (as) lead the Sahabah one night in Ramadan, and that he (sallallahu’alayhi wasallam) also lead them -on that night- in three rak’ahs of Witr.
Umar (ra) instituted the 20 tarawih with witr which became a matter of consensus.
Outside Ramadhan, there is simply no evidence for it.