FWIW, its use has grown over time, with new applications most significantly including MLIR (Multi-Level IR compiler framework, which allows you to define your own intermediate representations as well as compiler passes operating on them), https://mlir.llvm.org/docs/DefiningDialects/Operations/

The docs have been improving, https://llvm.org/docs/TableGen/

I also like these resources:

Lessons in TableGen FOSDEM 2019; Nicolai Hähnle https://fosdem.org/2019/schedule/event/llvm_tablegen/

- What has TableGen ever done for us?, http://nhaehnle.blogspot.com/2018/02/tablegen-1-what-has-tab... - Functional Programming, http://nhaehnle.blogspot.com/2018/02/tablegen-2-functional-p... - Bits, http://nhaehnle.blogspot.com/2018/02/tablegen-3-bits.html - Resolving variables, http://nhaehnle.blogspot.com/2018/03/tablegen-4-resolving-va... - DAGs, http://nhaehnle.blogspot.com/2018/03/tablegen-5-dags.html