There's a .swf version accessible through the page source on Newgrounds (https://www.newgrounds.com/portal/view/721544).
https://uploads.ungrounded.net/721000/721544_monstermind.swf
Once you have the .swf file, you should be able to play it through Flash Player or some other appropriate program. End-of-life plans for Flash Player shouldn't matter as long as you keep an offline program around for this kind of thing.
Granted, I haven't tried out the version on Newgrounds, and since it was uploaded December 5th of 2018 and this site's version was apparently updated on April 2nd of 2019, there may be some changes. I don't know what they are; hopefully just minor bug fixes.
Oh, also someone farther down in the comments posted a Google Drive link with a downloadable version (.swf again). It also appears to be some kind of cheat enabled version, and comes with a .txt with a list of cheats. I haven't confirmed whether they work, though.