Is it possible to download/acquire iMovie 10.1.6?
Get iMovie 10 via iMovie 9

It turns out that if you have iMovie 9, the App Store will allow you to upgrade to iMovie 10 without the normal checks.

I'm running macOS High Sierra 10.13.6 and the App Store just let me upgrade iMovie a moment ago.

Get iMovie 9

# 1. Download the iMovie 9.0.9 update package
curl 'https://updates.cdn-apple.com/2019/cert/041-98154-20191017-1fbef983-8d87-49af-83b1-edecc007ce2e/iMovie9.0.9Update.dmg' -o ~/Downloads/iMovie9.0.9Update.dmg

# 2. Mount the dmg
open ~/Downloads/iMovie9.0.9Update.dmg

# 3. Expand (unarchive) the pkg
pkgutil --expand /Volumes/iMovie\ 9.0.9/iMovie9.0.9Update.pkg ~/Downloads/iMovie/

# 4. Rename `Payload` to `Payload.zip`
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 5. Unpack `Payload.zip` (it will get named `Payload 2`)
open ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 6. Move `iMovie.app` to the `Applications` folder
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload\ 2/Applications/iMovie.app /Applications/iMovie\ 9.0.9.app

# Enjoy!
open /Applications/iMovie\ 9.0.9.app


Get iMovie 10

Go into the App Store, see that the option now says "Upgrade".

In case when you can not upgrade iMovie because of information: "your system can not meet minimum system requirements" :

# 1. Check whether your App Store contains iMovie in tab "Purchased"
(You should see iMovie with button upgrade, If you don't see you should
turn on iMovie)

# 2. turn off iMove 9 and App store.

# 3. move to trash iMovie 9 

# 4. Open App Store, go to "Purchases" tab and click "Install"


Notes

I don't know if this is perfectly reproducible, but I know that I just did it and it worked on my machine.

Some things that may or may not be important (i.e. I may have triggered a glitch mode):


I had opened App Store to iMovie 10 before installing iMovie 9
I installed iMovie 9 twice as a matter of verifying the script above
When I went back to App Store it gave the option of Upgrade instead of Get
I clicked upgrade and it downloaded, but requires a restart that I haven't done yet
I have a number of other downloaded but not installed upgrades in my queue


I hope that helps and it's not just me.