Win11 is detecting a virus or malware is included in the 8.4.6 release from ITCH. 

Any way to safely fix this?

False positive. Try downloading again.

The error message received is "An error occurred during installation zip: unsupported compression algorithm"

Same error on my end. Win 10

Appears to be an error with the extraction of the game.htlm zip file.  

You can try getting a zip file from another site and use your own browser to download the zip file. 

It did work for me.

Yea ended up doing that as well, just like the convenience of 1 button play style now. Hope whatever it is gets fixed though.