@daira You mentioned that some non-Overwinter nodes might not receive the alert as they may have partitioned away due to the network preference code. The code does not guarantee a partition will happen so they could still receive the alert.
In any case, it looks like v1.1.1 nodes will stay connected to non-Overwinter compatible nodes due to this commit: https://github.com/zcash/zcash/commit/d670db7164c9c49eb43f10b40efc2adaa473422f#diff-9a82240fe7dfe86564178691cc57f2f1
This is unintended but does increase the likelihood that the alert will propagate to reach its intended targets. Also, for those nodes which are not reached, with or without the alert, those old nodes would face service disruption at Overwinter activation anyway, per the comment above.