While I dislike the feature getting removed, this argument does not hold up to scrutiny. They will remove using external SMTP servers to send emails from external, third-party addresses. Sending as another Gmail or Google Workspace address will continue to work, but that path is fully Google-internal.
Emphasis mine:
> You can continue to use "Send as" for any -->Google-hosted identities<--, including other Gmail addresses or aliases associated with your Google Workspace account. The discontinuation will not impact these addresses, which only applies to third-party, non-Google email accounts.*
Emphasis mine:
> You can continue to use "Send as" for any -->Google-hosted identities<--, including other Gmail addresses or aliases associated with your Google Workspace account. The discontinuation will not impact these addresses, which only applies to third-party, non-Google email accounts.*