Starting with the release of Unreal Engine version 4.19, the delivery process for the Datasmith plugin has changed. If you have an “Active” Unreal Studio entitlement (see below), Datasmith can and will be installed alongside the Engine automatically.
In the left panel of the Epic Games Launcher, you should see the subscription status listed for Unreal Studio:
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If you don't see the highlighted area listed, you may not have entitlements enabled for this account, so be sure to log in with an Unreal Studio-enabled account. If you need additional information about entitlements for Datasmith, see the Unreal Studio knowledge base.