I have been discussing with several customers on the upcoming change to block auto forwarding in Exchange Online. The announcement from Microsoft is for Roadmap ID 63831 and goes into effect September 1st 2020.
There is no impact on external forwarding in this update, however automatic forwarding will be disabled based on the policy in a future update currently planned for September 1, 2020 and we will communicate via Message center. Once the policy takes effect messages that are being automatically forwarded outside the organization will be blocked and non-delivery report (NDR) will be sent to the user.
If like many of my customers you have a legitimate reason to forward to external addresses you need to login to https://protection.office.com/antispam and change the “Outbound spam filter policy (always ON)”
The setting you are looking for is under Automatic forwarding and you should set this to “On – Forwarding is enabled”
Now if you want to be more secure you can create a new Outbound policy and scope this to just the mailboxes that need forwarding enabled.