When you have the maximum amount of sent friend requests, adding another friend in game using add recent opponent as friend causes the game to crash.
Side note: Sent friend requets never expire. Kinda weird design imo but I dont work there.
You can remove old requests using the battle.net app menu.
Friend Settings → View Sent Friend Requests
Yeah when I did that i saw requests from 8 years ago. Seem like an unnescessarly long time to store it.