UPDATE: April 16, 2019: As expected, the story below was a false alarm. The information from MSI's German support team was inaccurate.
MSI's German support team told a Redditor that the company's B350 and X370 motherboards will not support AMD's upcoming Ryzen 3000 series processors. Despite AMD's promises about backwards compatibility, the e-mail states you will need a X470 or B450 based motherboard to support the new Zen 2-based processors.
This sparks speculation that MSI is being greedy by forcing customers to upgrade to a newer motherboard if they want to upgrade to AMD's upcoming processors. We'll have to wait to see if this is true or just an unfortunate error from the support team.
Other motherboard vendors could follow MSI's suit as a representative of another motherboard vendor, on condition of anonymity, told TechPowerUp that "Zen 2" processors have steeper electrical requirements that 300-series motherboards don't meet. This is an excuse similar to the one Intel gave for the planned obsolescence of its 100-series and 200-series chipsets, even as it was repeatedly proven that those motherboards can run and overclock 9th generation processors with custom firmware just fine.
I guess we'll learn more in the coming months but it would be sad if this were true.