About Expansion of the number of restricting domains for external sharing
According to the following knowledge base, you can configure up to 1000 domains to allow or block sharing of the domain list at the tenant level.
We hope that you can expand the number of domains that can be registered and make the number of domains unlimited
"Domain sharing restricted by SharePoint Online and OneDrive for Business":
We want to make sharing easy and enable automation in an allow list environment. Right now this limitation impacts our ability to truly roll external sharing out to the entire company. We need a minimum of 5000 domains, but would love to see 10000.