[Machine] Witr is not obligatory, but it is a sunnah established by the Messenger of Allah ﷺ. This is the statement of Ubada ibn as-Samit and ibn Abbas, and all of that is in addition to the other narrations mentioned regarding its place under the section of voluntary prayers.
الْوِتْرُ لَيْسَ بِحَتْمٍ وَلَكِنَّهُ سُنَّةٌ سَنَّهَا رَسُولُ اللهِ ﷺ وَهُوَ قَوْلُ عُبَادَةَ بْنِ الصَّامِتِ وَابْنِ عَبَّاسٍ وَكُلُّ ذَلِكَ مَعَ سَائِرِ الْآثَارِ الْوَارِدَةِ فِيهِ مَوْضِعُهَا بَابُ صَلَاةِ التَّطَوُّعِ