And all explicit lore sources point in that direction.
Cdev :
" Free will is one of the cornerstones of Forsaken culture, with the great capacity for both good and evil that it entails. "
Some text from the Forsaken intro quests in Cata :
“ You are no slave , . You are free to follow whatever path you choose from here.”
“Valdred will be a valuable asset to the Forsaken. As for the other two… there is not much we can do. We cannot force them ”
"You say she ran away? What a shame.
Very well. As long as she’s maintained her free will , there’s still hope. "
All the context needed for Baines comment here :
" You raised him as a Forsaken. But you planned to deny him his free will. To violate his mind. "
Lilian Voss on Zellings death :
" We Forsaken are raised with free will. That’s what separates us from the Scourge. Sylvanas Windrunner used to take pride in that distinction.
But after all I have seen in this war, I must wonder if that still holds true. "
So, I think it is pretty clear where the narrative intends to push us in interpreting these events. Proudmoore was raised as an undead capable of free will by Sylvanas, but was denied his ability to choose in contradiction to previously stated (both by Cdev and Cata intro quests) Forsaken values.
The only, and I mean only rebuttal to this seems to be oddly specific qualifications and interpretations of lore which are no where explicitly confirmed or stated by any game text. Sylvanas doesn’t deny raising him as a Forsaken. The players loyal to her do. The game doesn’t state that there is some unique clause for free willed undead raised by the Forsaken. Only players loyal to Sylvanas do. Indeed, in the Cata quest text it seems to imply undead raised who refuse to join the Forsaken are allowed their choice, “We can not force them.” Of course, they tend to be killed. Still, a choice is given and stressed as important.
So, to repeat myself, the fact remains Derek Proudmoore was raised as a fully functioning free willed undead by Sylvanas but denied the choice to become Forsaken when he was shackled immediately upon being raised, thus violating the cornerstone of what differentiates the Scourge and Forsaken.