People tend to refer to the bit where Discord rewrote a bit of their stack in Rust because Go GC pauses were causing issues.

The code was on the hot path of their central routing server handling Billions (with a B) messages in a second or something crazy like that.

You're not building Discord, the GC will most likely never be even a blip in your metrics. The GC is just fine.

I get you can specifically write code that does not malloc, but I'm curious at scale if there are heap management / fragmentation and compression issues that are equivalent to GC pause issues.

I don't have a lot of experience with the malloc languages at scale, but I do know that heat fragmentation and GC fragmentation are very similar problems.

There are techniques in GC languages to avoid GC like arena allocation and stuff like that, generally considered non-idiomatic.