Sims could have multiple Best Friends in previous Sims games. The first time I realized it was an exclusive relationship in Sims 4, I was fulfilling that aspiration to become best friends with a spouse, and I got a notification that my sim had drifted apart from her other bestie, and it made me sad. Then I did it again accidentally when I didn’t realize the other BF had already had a BF and I wiped them out. So I don’t even bother with it anymore because I don’t want to steal anyone’s BF.