Is kneeling a construct of feudal relations from the Byzantine court?
The short answer
No. Even if it were, kneeling expresses humility in our culture. But kneeling as submission is far older than Byzantium: it appears throughout the Old and New Testaments — Jesus himself knelt to pray in Gethsemane.
Humility regardless
Even if kneeling came from feudal custom, it would still express humility in our culture.
Older than Byzantium
Kneeling as an act of submission goes back much further, in both Testaments. The old Catholic Encyclopedia notes that Jews knelt for solemn occasions, urgent petitions or fervent prayer, and that Genesis 41:43 and Esther 3:2 show kneeling was the accepted posture of suppliants in the East. Solomon knelt to dedicate the Temple.
In the Gospels
Jesus' great prayer for his disciples was offered looking up to heaven (John 17:1), but in Gethsemane he knelt to pray (Luke 22:41). Lepers knelt begging his mercy (Mark 1:40; see 10:17).