We discussed Chromium's current plan to support user scripts managers in Manifest V3 during our WebExtensions Community Group (WECG) session at TPAC[1] last week. The notes for that meeting haven't been merged yet, but there's an open PR[2] and when they are they will live here[3].
In short, the current plan in Chromium is to require end users and extension authors to opt into execution of arbitrary scripts via a Chromium UI setting and new permission, respectively. During the meeting Firefox folks raised some questions/concerns about this plan and it's probably best to try to align with them on next steps if possible.
And typing this out is making me realize we don't have a great tracking issue for this in the WECG repo. Just threw together a placeholder issue[4] to track discussion in this area.
[1]: https://www.w3.org/events/meetings/7bbba4a3-8305-45cd-a998-6...
[2]: https://github.com/w3c/webextensions/pull/277
[3]: https://github.com/w3c/webextensions/blob/main/_minutes/2022...