This message also persists in 17.3.3 version:
"%PLATFORM-1-NOSPACE: bootflash : no space alarm assert"
And I believe it's just a threshold set in the system, probably set in the code as a log message.
In my case was generated when the bootflash had about 28-30 MB free space left. After cleaning the bootflash of some old images and having about 1GB free space then another log message was generated to show that the state came back to normal:
"%PLATFORM-5-LOWSPACERECOVER: bootflash : low space alarm deassert"
