* Nim's current async implementation creates a lot of cycles in the graph, so ARC can't collect them.
* ORC is then developed as ARC + cycle collector to solve this issue, and it has been a success.
* This 1.4 release introduces ORC to everyone so that we can have mass testing for this new GC and eventually move torwards ORC as the default GC.
TL;DR: ORC works with everything† and will be the new default GC in the future. Your old Nim code will continue to work, and will just get faster‡.
† We are not sure that it's bug-free yet, which is why it's not the default for this release.
‡ Most of the time ORC speeds things up, but there are edge cases where it might slow things down. You're encouraged to test your code with --gc:orc against our default GC and report performance regressions.