The description of the Supporter pack says:
Discounted Access to the Finished Game (ETA: April 2020)
Will it come in a form of steam key or access to standaolne client from the official website?
Anyone who buys Last Epoch through our website has access to both.
That said, for purchasers of the $10 Ardent Traveler pack, the Steam key won't be delivered until you've game access.