This Block installed OK, and in fact still runs well in Moodle 3.1 - perhaps because it's operation is actually fairly simple and straightforward.  It still functions as documented, including the notification to Students (if used) and booking in Calendars correctly.  The only slight niggle is the formatting of the Block does not seem to adhere to the newer Block formatting (see below).Very easy to use :-). You can try this Block yourself for real on our public plugins site: h...

Perhaps because the last version on this Block was released for Moodle 2.4 is does seem to have some small formatting issues - in Moodle 3.1 at least.  The Date/Time selector overflows the Block area a little, although it doesn't seem to affect the functionality at all, just looks a little untidy.  

As this is a fairly old Block now (with no recent updates) and features a number of dropdown menus for selecting the Date/Time, it might not pass some of the more stringent accessibility standards now in operation?

I've not reviewed code.  However, based on the fact the last release was for Moodle 2.4 it's quite possible it doesn't comply with current standards.  However, it's unlikely to result in any security issues as far as I can see given that it's a course based Block.