Final reply from sendgrid:
Hi there,
The Shared IPs on our Free, Essentials, and Basic level plans may end up on blocklists as they are part of the nature of Shared IP Pools. Many other senders are using the same IP addresses in the shared IP pool. By sharing IP addresses, when an IP address in a shared pool is blocklisted (this can be for many reasons, including a sudden increase in sending volume), it will affect everyone using the Shared IP Pool containing the blacklisted IP Address. After IP Pool transition, this blocklisting issue could still be present, but should not negatively affect sending, if at all, for an extended period of time.