Canceling King Arthurs boosted attack on cards with no effects
From Jenn S. at Restoration Games

Essentially, Arthur's special ability is an effect that can get
canceled, so the Boost would be discarded.
There is some further discussion on it in this BoardGameGeek thread:
Feint vs boosted Excalibur.

TL;DR even with Excalibur, the boost would be discarded if an opponent's card discards effects.