BT has quietly decided to shut down its high-profile Web21C development platform in favor of new technology it acquired in its recent $100 million acquisition of Ribbit.

BT said it plans to completely shut down support for the current Web21C SDK on October 10, resulting in strong complaints from existing Web21C developers, who say they feel abandoned by t he BT move.

Developers on the Web21C forum boards have even made a call to take over the Web21C APIs, possibly as an open source project. But given how closely those APIs are tied to BT’s network, it’s not clear how such a move would work.

That BT would favor its newly-acquired platform isn’t surprising, but shutting down support for the SDK entirely – and on short notice – has drawn the ire of developers who had built working applications relying on the Web21C platform.

The closure was announced in a blog post last Thursday by JP Rangaswami, Managing Director, Strategy & Innovation for BT Design and confirmed by a BT spokesperson this morning. BT wasn’t available at this time for further comment, the spokesperson said.

Ribbit’s platform consists of largely client-side APIs based on Adobe Flash and Flex components and libraries. BT originally said that Ribbit’s client-side technology would complement the largely server-side components of its Web21C platform, but decided instead to invest all of its future time and effort in the Ribbit platform. That includes “migrating the Web21C SDK into Ribbit,” wrote Rangaswami, essentially providing similar REST-based server-side APIs on the Ribbit platform beginning in December of this year. Developers will be able to write applications to these APIs in a variety of languages, BT said, including Python, Perl, Java, Ruby, and others. Those server language binding capabilities will be rolled out in 2009, BT said.