Please go to Microsoft 365 admin center -> Security -> Audit -> Classic Search -> In the Activities, choose Changed a sharing policy to check changes of external sharing settings.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.