[coderdd]

The backend is open sourced, it is Kythe.io. It supports go, c++, java out of the box, for some definition of out of the box. Maybe even typescript. Also cross-references protobufs work generated code of you make the stars align ;)

As for UI, treetide/underhood I mention elsewhere is the only open option now.

But Kythe comes with command line utils and an API you can query directly as well.

What is missing from the open source is a production-ready parallel serving table builder. There is one in golang which uses Apache Beam, but last time I checked the go workers are not well supported on the Flink runner. It didn't even work properly on the GCP runner. Hope this would change.