After about a year of experimenting, I'm unstickying and locking this thread. The local file/folder saves feature is now enabled by default in C3 r219.2+ with Chrome 86+ and will make its way through the beta cycle to the next stable release like any other feature. The related origin trial is now over so there should be no further disruption to the availability of the feature.
For the record, some further issues came up in beta releases with saving folder projects, so I've decided against merging the changes back to the r218 stable release. The fixes ought to have the full beta cycle to verify them. So this will (finally) ship enabled-by-default for everyone in the next stable release.
This thread has served its purpose for being a place to notify people about an experimental feature and discuss it or any changes to it. I'm also closing it because threads like this tend to accumulate random support request replies in the long-term, and this is no longer an appropriate place for those kinds of posts - if you have any further questions or help requests for this feature, please start a new topic as you would do for any other Construct feature.
Thanks to everyone who helped test this feature! It's been a bit of a bumpy ride at times but it's a significant step for the web that browser apps like Construct now have access to the file system. This makes it possible for the first time to provide features like folder projects in the browser. Previously this was only possible with native apps (like in Construct 2), and this step is another sign of the continual progress of the web platform. We'll be running more experiments in future - I'll start separate sticky threads for each of those in a similar manner as we did for this thread. Other experimental features should be shorter-lived before shipping enabled by default - this one was unusually long given the size and scope of features covering file system access in the browser. So, stay tuned on the forum for more experiments!