SCIM was the de facto Linux input method framework for a quite long time before IBus took over its place in 2008. SCIM core framework was developed by James Su at SUSE, which was aimed at supporting as many languages as possible. It was a success and currently it's still the only framework supporting some rarely used languages. The SCIM core framework has support for XIM and GTK+2.
The design of SCIM is simple, which was basically bundle most functionality into the core application even though it allows some engines to be developed as plugins. As there were some issues discovered to be relevant to C++ ABI transition, scim-bridge is developed to work around the problem. scim-bridge supports GTK+2, Qt3 and Qt4 (added lately), but Qt4 support was reported to be not promising, and there are GTK+3 patch around according to its homepage.
The project is declared as maintained by several hardcore SCIM users currently, who has also admitted that they are lacking of specialized individuals to explain and/or resolve some issues. But the team has been trying their best to make the project buildable and installable.
Some distributions have intentions to drop SCIM support in near future because it's lacking upstream support and already replaced by other frameworks in most cases.