The update is available on the Microsoft Update Catalog so it has not been "pulled." The link is above. KB4458469 is part of a set of cumulative updates for versions 1803, 1709, 1703. The changelogs are very extensive. There is no reason to connect it
with the sign off version for 1809.
The update is part of the preparation for the new build. IF it has been pulled, some problem was found. Whether that problem is actually tied to the new build, is not for me to know for sure. If 17763 will be the RTM, then when that occurs, it is no longer
just part of the Insider program. I will assume it will take a few weeks for the RTM to go public, so plenty of time to repair a problematic update.
I suppose, from the responses, no one has heard anything one way or the other concerning the update. When an update is no longer available for the normal Windows update process, to me, it has been pulled. Has it been removed from the catalog, of course
not since many folks already have it on their systems. If you do a manual update, then Microsoft can deny any responsibility if problems did occur.