One of the the debugger’s main purposes is to show variables and object permissions.
Many full games made in GDevelop have hundreds of variables (I’ve worked on a project with over 1000 globals), automatically displaying them on the screen would be impossible.
That said, if you want to display specific variables, you should just make a single or set of global “debug” text objects, and then a external event sheet that populates those objects with the variables you care about, then you can just include that in whatever scene you need it in.
No reason for a separate feature that would be problematic for many games.