I think that's mostly done in organisation where there's time, budget and willingness to optimize as far as possible.

Sacrificing the guardrails doesn't make sense for the "general public" software but does tremendous sense in environment where latency is critical and the scale is massive. But then again, in those environments there are people handsomely paid to have a thorough understanding of the software and keep it working (making updates, implementing features etc).

I worked on a software that was written to be garbage-free whenever it could. Latency (server-side latency, i mean) in production (so real-world use case, not synthetic benchmark) was about 7-8 microseconds per request (p99.9) and STW garbage collection was at around 5msec (G1GC, p50, mostly young generation) or ~40 msec (p99.9, full gc) and was later lowered to ~800-900 microseconds with ZGC.

I know it might sound elitist but the real difference here are... Skill issues. Some people will just shun java down and yap about rewriting in rust or something like that, while some other people will benefit from the already existing Java ecosystem (and tooling) and optimize what they need to get the speed they're targeting.

I know I'll be downvoted by the rust evangelism task force, but meh.