Chisel absolutely isn’t the type of software that benefits from upgrading because it’s largely standalone. They could be the last project still stuck on 2.x a decade from now and it wouldn’t make much of a difference to its users.

I’ve only used Chisel for a few projects but I’ve never used anything but Chisel in those codebases. Simulation, verification, and all the painful stuff in FPGA/ASIC development depends on non-Scala tooling and all of the inputs (parametrization) are just read in from JSON files produced by scripts in other languages.

It would be nice to be up to date but the hardware NRE is so damn high that working around any limitations in Scala support is a rounding error. Chisel’s outputs are sent out for $X00,000 fab production runs so no one gives a damn whether it’s Scala 2 or 3 as long as it ships a working IC. The last time I used Chisel I was working on a mixed signal design where the Synopsys Fusion Compiler (maybe Custom compiler?) licenses alone ran into the hundreds of thousands per year (iirc it was per seat, so we must have spent over a million per year on Synopsys alone).