The latest update (v15) mentions improved compatibility with PHP 7.4, so indeed it would seem MachForm is not yet compatible with PHP 8. But then it's also worth noting that even some rather prominent PHP applications (MediaWiki, for example) do not yet have PHP 8 compatibility.
That being said, I should think MachForm is soon enough going to have to drop support PHP 5.5 and move the minimum version requirement to something like PHP 7.3, in line with the bulk of modern applications, and will undoubtedly need to support PHP 8.
