>What programming language stack do you use that’s closed source?
Verilog/System Verilog (generally HDL/FPGA/ASIC design/verification).
Yes, it's a niche (some may even say it's not software, it's hardware, but it is indeed a large code base that requires a software engineering environment/toolchain/practices/etc...). Again, just answering your question for the benefit of the discussion. It would be interesting to hear of others such closed source areas.