TLDR; If you used the Incoming Webhook urls generated before February 2021 you should have updated them before the 1st of July 2021 PST or they won't work anymore. You should see a message there "You must update this connection to keep using it after June 30, 2021. ".
If your teams uses Microsoft Teams Incoming Webhook for integrating with another system, you should have updated those to be tenant specific. The ones that require update are highlighted in red.
When you click on it there is a bigger red message near the url indicating the requirement to update.
If you don't notice those your Microsoft Teams Incoming Webhook Url stopped working. Before you should see a warning from Microsoft "You must update this connection to keep using it after June 30, 2021" like below.
If you don't update most of the responses result in the following error: Microsoft.Griffin.Connectors.Store.ExchangeInvalidGroupIdException: We can't find that group. It may have been deleted. Please verify whether the group exists. ErrorCode:ErrorInvalidGroup