[BUG] Optional dependencies aren't optional for npm 6 (backwards compat.) if npm 7 regenerates the lock

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

On a system still running npm 6, no install or ci commands can be executed if a dependency has an optional transitive dependency, and the current v2 package-lock.json was generated by npm 7, and package-lock.json has been recreated.

npm 6 would include the optional dependency within package-lock.json's dependencies section with optional: true. npm 7 seems to do this when installing the package originally, but if the lock file is deleted npm 7 then won't do this, but still references the optional dependency within the requires section of other dependencies. When npm 6 comes to install using the lock it can't find the optional dependency and errors out:

Expected Behavior

Based on the documentation I can find, npm 7 should (always?) keep a backwards-compatible package-lock.json so that npm 6 can still install dependencies in a mixed project.

Steps To Reproduce

1. On a system running npm 7, create a project and install jest-haste-map. This has an optional dependency on fsevents but that's Mac only. (Note: I've been running on Windows and Linux, not sure if a Mac would fail to reproduce.)
2. Delete package-lock.json. We did this because of the mess that mixing npm 6 and 7 did during merges and it was easier to delete the whole file than to resolve Git merge markers.
3. Run npm install again. npm 7 should regenerate the lock, but it won't have put an entry for fsevents in the parent dependencies object.
4. Now move to a system running npm 6 with the same project (package.json and the new package-lock.json).
5. Run npm ci (or even npm install).
6. See an error about fsevents not being accessible from jest-haste-map.

Environment

• OS: Windows 10 1909 amd64
• Node: 12.18.2
• npm: 7.11.2 (just tested on 7.12.1 too)

Start by reproducing the issue with package.json, a regenerated package-lock.json, jest-haste-map, and its optional fsevents dependency. Compare npm 7.11.2 or 7.12.1 output with npm 6 using install and ci; done means npm 6 can install successfully from the regenerated lockfile.