Roy Smith wrote in
:

From a practical point of view, it means that if the glide slope
transmitter were to fail, ATC would not know about it (and thus not be
able to warn you about it). The fact that it's got a daily time
window makes me think that the monitor alarm must be located in some
tower which closes down for the night so there's nobody around to
hear/see the alarm. It's curious that just the glide slope (and not
the localizer) is subject to this.

From a legal point of view, it doesn't mean anything to a part-91
operator. I believe part 135/121 guys may not be able to file that
approach as an alternate, or something like that.

Yep. If it's unmonitored, we can't use that approach when determining an
alternate. We've had that problem at KGLS for a long time. The ILS is
unmonitored, because the light is in a closet somewhere in HOU Approach,
not in view of the controllers. Apparently there is no way to get it
moved. Thus, we can't use the ILS approach for alternate determination.
The only approach available is the VOR, which makes alternate minimums
there higher. We can still use the airport for an alternate, but the
weather minimums are higher, because we can't use the ILS approach.