The TLDR is that the GC did cause problems so we had to avoid it for the block cache. Luckily we were able to do so without exposing the complexity in the API. Not for the faint of heart. Don't try this at home kids.
Any reason why the block cache needs to be 10s of GB in size? Cassandra, for example, usually has a rather small key cache on heap, and then just relies on the kernel page cache. I don't have experience with cockroach, it looks like the block cache is similar to cassandra row cache, which can be configured to be on heap or off heap, but usually not beneficial to enable.
Would be interested to see if the garbage collector has presented any problems when running in production