Chrome OS users may finally be able to sideload Android applications soon, following some changes that were recently noticed in the feature’s Chromium bug tracker report. It's also not likely to arrive soon, if at all, with the most likely timeframe appearing to put the feature in Chrome 77 in August.
The addition of sideloading would essentially bring Chrome OS the Android-specific ability to load an application from a secondary source other than the Google Play Store. Right now, that isn't possible and no option exists either unless the user happens to be in the Developer Channel.
The bug in question centers around an option in the Settings app that would allow apps from third-party sources, almost exactly as is seen on Android.
At least one comment on the tracker now notes that the capability won't arrive in version 75 and the features listed as of Chrome 76's feature freeze on May 17 don't seem to include sideloading. Regardless, the feature itself has been set to priority 1, which is only a step below the most urgent level in the bug tracker's three-step priority rating system. So it does seem to be on the way.
...maybe, if history doesn't repeat itself
Android application sideloading in Chrome OS is a feature that's been in the works since 2017 when it was added to the bug tracker as a "priority 3" bug. Later on, within a month's time, its status was changed to "won't fix" as the result of the tradeoff it would present in terms of security.
The long timeframe involved here leaves open the possibility that the feature will be de-prioritized again if the tradeoff with security proves too much and other changes don't offset the risks.
Chromebooks can now download and run the Android Studio IDE as of Google I/O 2019 but there's no way to natively test their apps on Chromebooks. Since the IDE for Chrome OS is still in a beta form, for now, that doesn't presently include the suite of emulation tools available on other platforms, making testing apps built in the developer tool an arduous task at best.
Sideloading apps would be a way to step around the issue but there's no guarantee that will still be considered a top priority at all if Android Studio is updated to support native testing or emulation in the meantime.
Now, Google has implemented dozens of changes over the years to make Chromebooks even more secure when running apps or accessing the web but there aren't necessarily any big changes that would make sideloading much safer. Conversely, allowing Android apps to be loaded outside of the Play Store would still appear to be a detriment to Chrome OS's top selling point, security.
For clarity, Chrome OS has so far functioned in a way that's more akin to iOS than Android's wide open ecosystem. Adding the ability to sideload applications changes that, making this a much bigger deal than it might seem on the face of it. By adding this across the board, Google is opening up what has effectively been a well-sandboxed walled garden.
Google could ultimately house the capability behind a flag or other settings not immediately accessible to the average user but that doesn't seem to be in the cards for the time being.