In https://google.aip.dev/129#rationale-for-normalizations it is stated:

For the email ada@example.com a client may choose to specify ADA@example.com, aDa@example.com, or AdA@example.com. These are semantically identical and should all be accepted by the service. The service then may choose to normalize the email address for storage and retrieval through downcasing or canonicalization.

Considering this is up to the provider to decide the statement seems dangerous. That is not common but I would guess there are systems out there making that distinction. RFC5321 [1] states :

The local-part of a mailbox MUST BE treated as case sensitive.

[1] https://datatracker.ietf.org/doc/html/rfc5321#section-2.4