Note: Apple has now released fixed versions of the Security Updates: details here.
Apple has overnight pulled one of the two BridgeOS updates required to support its recent Security Updates for Sierra and High Sierra and the 10.14.6 update.
Further examination suggests that this is the version of BridgeOS or iBridge to support the Sierra and High Sierra security updates. The version to support the Mojave 10.14.6 update remains available. Consequently, it should be ‘safe’ to update to 10.14.6, but not to apply either the Sierra or High Sierra security updates, which in any case aren’t available at present. Further details are here.
Thanks to Al Varnell and Mr. Macintosh for valuable information about the BridgeOS situation.
Revised 2110 UTC 29 July.