My infrastructure guy just install the new BC with CU 10 (August 2019), I double-checked and its runtime is 2.4
I even get the below error when I try to install an extension with Runtime 3.0:
The runtime version of the extension package is currently set to '3.0'. The runtime version must be set to '2.4' or earlier in the app.json file in order to install the extension package on this platform.
based on this link, if I'm on CU10 then it should give me the runtime 3.0
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-json-files