This Jira issue introduced validations on multiple configs. As a consequence, config security.protocol now only allows upper case values such as PLAINTEXT, SSL, SASL_PLAINTEXT, SASL_SSL. Before this change, lower case values like sasl_ssl, ssl are also supported, there's even a case insensitive logic inside SecurityProtocol to handle the lower case values.
I think we should treat this as a regression bug since we don't support lower case values anymore since 3.3.0. For versions later than 3.3.0, we are getting error like this when using lower case value sasl_ssl
Invalid value sasl_ssl for configuration security.protocol: String must be one of: PLAINTEXT, SSL, SASL_PLAINTEXT, SASL_SSL