This blog is in continuation of previously discussed topic around discontinuing the unlimited text data types. You can get a background here that was published in 1811 (November 2018) release.
With solutions created from May 2019 (1905 release), you are no longer allowed to use unlimited text types in the XBO/CBO definitions. For solutions created prior to 1905, the usage of these type of fields would continue, but we recommend that you use the String data type or text data types of limited length in such solutions as well. You can find the data type in the http://sap.com/xi/AP/PDI/bo namespace.
With solutions created from May 2019 (1905 release), unlimited text data types will not be available in code completion (CTRL + SPACE) while defining fields or place holders for message definition. If you use these data types in the XBO/CBO, you will get compilation error messages as shown in the following screenshots for different scenarios.