Please let me know if you are able to get a resolve to this question (via any means including outside of this forum). I too am running El Capitan v10.11.6 and need to install a version of iMovie that will work with no issues. Would even consider installing ANY older version of it will work OR would like to find a source where v10.2.1 can be downloaded SAFELY (100% clean).
Trust me, you're not alone. Many people like you are searching the Internet for "where can I download an old version of imovie" or "where to download iMovie for Catalina", especially there are big changes on the new released iMovie, e.g. from iMovie '11 to iMovie 10. The reasons may include, "my favorite feature has gone", "compatibility issue", "imovie not working", or "imovie can't import my video", etc.
As all of you may not have come here for downloading mac os x el capitan iso and installing it on your pc, we understand your interests that lie in getting a sneak peak inside the mac os x 10.11 to find out how it used to look.
Mac OS X El Capitan 10.11.6 is a powerful release of Mac OS X from Apple Inc. This free download of El Capitan is a standalone Disk Image InstallESD DMG installer for Macintosh based desktops and servers. 2b1af7f3a8