Here are the breaking changes for 8.0.
Changes in Logstash Coreedit
Ruby Execution Engine removededit
The Java Execution Engine has been the default engine since Logstash 7.0, and works with plugins written in either Ruby or Java. Removal of the Ruby Execution Engine will not affect the ability to run existing pipelines. #12517
Support for UTF-16edit
We have added support for UTF-16 and other multi-byte-character when reading log files. #9702
Field Reference parser removededit
The Field Reference parser interprets references to fields in your pipelines and plugins. It was configurable in 7.x, with the default set to strict to reject inputs that are ambiguous or illegal. Configurability is removed in 8.0. Now Logstash rejects ambiguous and illegal inputs as standard behavior.