Given what you’ve said previously about the relative lightless of plate armor, is the idea of the French knights being encumbered by their armor and outmaneuvered by the lighter longbowmen at the battle of Agincourt a myth?

Not a myth, but an oversimplification. What happened at Agincourt was this:

1. Henry moved his men into a position from which they could neither be effectively flanked nor charged by cavalry, and which would impede the French in their efforts to make use of their cavalry and their artillery.
2. There was a failed French cavalry charge which, along with the copious rain, turned the space between the French and British lines into a morass of deep mud. 
3. The French men-at-arms then had to march on foot through that morass for about 300 yards while getting shot with arrows. Now, people debate all the time about how effective these arrows would have been in penetrating plate, but at the very least, the threat forced them to slow down and walk with their heads down to prevent themselves from being shot in the face, further extending their slog.
4. The French men-at-arms then hit the English men-at-arms, and had some success, but this is when the English archers counter-charged and absolutely massacred the French men-at-arms. 

I don’t think the armor was the major factor as much as the exhausting mud march and then getting flank/rear-charged by fresh troops when the terrain has squeezed you into a narrow space where you can’t react effectively.