I stand corrected

EDIT: Nope, it was CGI.

For the opening scene, 11-year-old Luke Kokotek served as the body double for Noah Schnapp, who plays Will.

Deadline reported that the services of Weta FX were used to de-age Schnapp, following which his face was composited onto Kokotek.

https://thecinemaholic.com/will-ai-deaged-stranger-things-season-5/