Flarum checks the requirements during installation, the code can be seen here flarum/frameworkblob/v1.8.5/framework/core/src/Install/Installation.php#L103-L122
Are there any requirements missing, or are you just referring to a the absence of a standalone tool?
Sure, it's not very efficient to download the entire Flarum just to run the check, but bandwidth is cheap compared to the free time of the developers needed to maintain a separate tool, not to mention the work to make sure it's kept up to date with the main code.
Maybe someone can write a tool that just syncs that part of the Flarum codebase into itself, that way it's guaranteed to be up to date.
There's an added complexity with Composer, Composer checks some of the system requirements of Flarum, its dependencies and extensions. Those requirements might vary depending on the PHP version. So here again, you need the whole Composer client to test compatibility, and while doing that Composer will already have to download Flarum in its entirety. So you might as well open the built-in installer to check the remaining requirements.
I personally don't see much value in a standalone tool, even if it can be made. Flarum requirements are all very standard. The most uncommon requirement is Composer and optionally SSH, and you shouldn't need to download a separate script just to know that. Apart from that, all webservers should meet the requirements. So you can decide to move on to installing Flarum without any more concern. Then if you have any module disabled, it will tell you during installation and you can just enable it before continuing.
Maybe the only real dealbreaker that needs checking are the database engine and version. That part is sometimes difficult to find in a managed hosting, so it might make sense to check this before downloading Flarum. A script or SQL command might be needed to find out. But here again, the hosting is ANCIENT if it doesn't meet the requirement 😅
