After changes in HHH-9894 Closed , Informix Boolean type is supported. Hibernate InformixDialect declares support for Informix version 7.3, when this data type was not supported.
Guide to SQL Syntax Version 73

I propose SMALLINT data type column mapping support for Informix 7.3 and legacy database schemas via a variable as in SAP HANA Driver ("hibernate.dialect.informix.use_legacy_boolean_type")