Seems to be available on APK pure (both free and paid) but that depends on if you trust that site and if it can even run on modern android versions. Seems you can also find an email at the bottom on Jason's Enjoy Sudoku website, emailing them might be an option. You could try crawling through Github, maybe there's an old repo or something under a different name there.

Not that it’s the same thing, but I wrote a little Easter Egg into https://port87.com/ where in the settings page you can play sudoku. It’s got different levels of difficulty, and I’ve never been able to beat a hard difficulty game.

I had it forever, but it doesn't work on later Android versions. I bought the paid version too.

Apk Pure has been mentioned, also try APKmirror.com

After looking at the game via the link you provided, I'm gonna suggest https://f-droid.org/packages/org.secuso.privacyfriendlysudoku/ if you can't find Enjoy Sudoku.

On the play store, it's usually possible to download old apps even long after they're no longer offered.

In the play store:
Click on your User icon -> manage apps & device -> manage
Then change the "This Device" selection to "Not Installed" and scroll to whatever app you had installed at one point.