From BotBuilder

This new iteration of Teams AI has been rebuilt from the ground up. To easy the migration process we have created a plugin @microsoft/teams.botbuilder, which allows you to use a botbuilder activity handler and adapter to send/receive activities through our new abstractions.

Why A Plugin?

Because by using a plugin we are able to leverage all our new features while allowing developers to easily and incrementally migration activity handlers from the legacy activity handlers to our new App class handlers.