The latest news on this week’s macOS updates is that the Mojave update to 10.14.6 does appear ‘safe’, but the two security updates 2019-004 to Sierra and High Sierra remain unavailable, and shouldn’t be attempted until Apple replaces them.
The problems appear to be in the BridgeOS update, which is specific to Macs with T1 or T2 chips. The version of BridgeOS or iBridge, 16.16.6568.0.0, which ships with these updates appears to work correctly in the 10.14.6 update, but can cause problems with sleep and wake in the Sierra and High Sierra Security Updates 2019-004.
At its worst, this results in the updated Mac suffering a kernel panic either when it tries to sleep, or when waking from sleep. Disabling system sleep is a workaround for the time being, which probably isn’t too disruptive for desktop Macs. However, most users of MacBook Pro and MacBook Air models expect their computer to sleep when they close its lid, and to wake on opening. If either of those actions cause the Mac to reboot because of the kernel panic, this is extremely disruptive. The same basic workaround can be used, to shut the Mac down every time rather than let it sleep.
MacDailyNews Take: Currently, as Oakley notes, the macOS Sierra and High Sierra Security Updates are not available, so an attempt to install an old download should fail, as the Mac can’t obtain the BridgeOS update for it. If you aren’t ready or able to update your Mac to Mojave, you’ll have to wait for Apple to issue new Security Updates for Sierra and High Sierra.