An email that has supposedly come from MSI's German-language tech support team has stated that the company will not be offering support for AMD's upcoming line of Ryzen 3000 Matisse desktop processors on older motherboards (300-series). However, there has not been an official statement from the Taiwanese company in regard to this issue yet.
https://www.notebookcheck.net/Alleged-MSI-tech-support-email-states-older-300-series-motherboards-won-t-support-AMD-Ryzen-3000-processors.417490.0.html