This document will be updated in the v1.12 release to be explicit about the SDK support for a runtime.
The topic is about Dapr SDK forward compatibility - "Can I remain on an older SDK and move the runtime from v1.9 to v1.11?" for example. Yes you can. Newer Runtimes should not break older SDKs
Then there is about Dapr SDK is backward compatibility. "How many previous versions of the SDKs does a given runtime support". Currently we test for 2 versions: the current SDK and the previous one. However given the non-breaking changes the previous 3 SDKs should be supported for a new runtime. We are going to make this change for this upcoming release explicitly in the v1.12 testing and documentation here will reflect this.