According to the standard v1.1 (https://www.inform-fiction.org/zmachine/standards/z1point1/sect11.html ) bit 1 in flags 1 means " Status line type: 0=score/turns, 1=hours:mins" in v3 and something else in v6. The meaning in v4 and v5 is unspecified.
The Inform 6 compiler seems to use this bit to signal that the statusline should show time, even in v5, and the I6 library reads the bit to check if it’s supposed to show time.
How safe is this? I imagine there could also be interpreters which set the bit to zero before the game starts.