I had taken a quick look back through evolution of Unity's ML-Agents from initial prerelease announcement to the present day. The features have been good, but looking at the dates highlighted a conspicuous gap: there have been no releases or announcements in the second half of 2021. This is notable because ML-Agents had been on a rapid releases schedule, with versions coming out every few weeks, since the original prerelease announcement. But things had come to a screeching halt -- why?

[Update: Release 19 became available on 2022/1/14, but I have yet to find information on why there was a long delay between 18 and 19.]

In the absence of official announcements, I went poking around through other publicly available information. The first resource was a brief look at the GitHub commit history for the ML-Agents repository. We've been warned the "main" branch can be unstable and I thought that meant it was the active development branch. This is apparently false, as I found hints of additional working branches that are out of public view. For example, the original commit for "Deterministic actions" mentioned "release 19" even though the latest release is currently 18. These mentions of "release 19" had to be removed as merge #5637. In the description, it mentioned the branch "release_19_docs" which is not visible to us.

With development progress partially obscured from view, I moved on to looking at currently open GitHub issues. When doing a first glance at an unfamiliar repository, I first look at the most recent items (the default view) and then I sort by "Most Commented" to see the most popular items. Nothing caught my eye as a likely reason. However, it did eliminate the possibility that Unity had killed the project, because team members are still responding to issues. So that's comforting.

The next resource was to browse Unity Forum for ML-Agents. Scrolling through issues between June and now, I saw multiple references to memory leaks. ("Unity ML 2.0.0 Memory leak" "Training is more and more slow" "Memory leak using imitation learning") Returning to issues list, I saw #5458 "Memory Leak" is currently open for investigation. However, there's no comment one way or another if this bug (or another) is holding up release 19, so my quest came up empty-handed. However, I did get the idea if I run into a memory leak with release 18, I can try downgrading to release 17 to see that helps. Plus I came across a bunch of other interesting pieces of information.