Why McKenna Grace wasn’t in final season
Exec producer Steve Holland said Grace’s character Paige Swanson was very popular, even though she was only in a few episodes starting in second season, and fans keep asking why she didn’t appear in the final season.
But he said she wasn’t brought back because they felt her story arc didn’t need closure. Also, Grace is such a sought after actress these days they probably couldn’t get her even if they wanted to.
https://tvline.com/interviews/young-sheldon-paige-what-happened-to-her-big-bang-theory-1235239971