adding the list back and adding a note about downlevel-dts

docs/typescript/design.md

@@ -25,6 +25,11 @@ Use [caniuse.com](https://caniuse.com) to determine whether you can use a given
 
 {% include requirement/MUST id="ts-support-ts" %} compile without errors on TypeScript v3.5 and greater.
 
+Supporting older versions of TypeScript can be a challenge. There are two general approaches:
+
+1. Don't use new features.
+2. Use [`typesVersions`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-1.html#version-selection-with-typesversions), which might require manual effort to produce typings compatible with older versions based on the new typings. Much of that effort can be automated using [`downlevel-dts`](https://github.com/sandersn/downlevel-dts).
+
 {% include requirement/MUST id="ts-register-dropped-platforms" %} get approval from the [Architecture Board] to drop support for any platform (except IE11 and Node 6) even if support isn't required.
 
 ## Namespaces, NPM Scopes, and Distribution Tags {#ts-namespace}