GTK_DEBUG set but ignored because gtk isn't built with G_ENABLE_DEBUG
 

Since gtk+3-3.24.14-x86_64-1.txz upgrade in latest current, I can no longer use GTK3 Inspector like this;

I am still on last week's -current, but the change appears to be a consequence of switching to Meson.

I think it is better to leave the GTK 3 debug features disabled. You can always build a private version for debugging.
Ed

Why do you think so?

The ability to peek into a program's internals opens up potential security holes.

It is not difficult to build a private GTK 3 and install it in ~/.local I have done that for debugging.
Ed

GTK3 inspector is a huge feature, that helps you create or inspect GTK3 themes, especially for us that creates themes and do this on a regular basis.

It is a letdown for all Slackware users doing this.

EDIT1:

I can see in the configuration that a flag called "--enable-introspection" exist, maybe it is enough just to enable this one?

The inspector is very useful for developers. For everyone else, it is a potential attack surface.
Ed

Thanks, I'll have that fixed in the next update batch.