Sorry, I actually went too fast, there are already two threads for this requests with many more votes 
@Emily_Roman or @Marie probably will merge yours with:
https://forum.asana.com/t/collapse-or-re-expand-all-sections-button-needed/
or
https://forum.asana.com/t/ability-to-expand-contract-all-subtasks-in-task-list-at-once/
You might want to go ahead and add your vote there as well
Take care,
Rosario