This word, spelled readonlyness, is used in the code of a project I'm working on. Readonliness seems to be a more logical spelling, as in ready - readily, beauty - beautiful, lonely - loneliness, etc. The question is, is the spelling with Y a mistake or is it an acceptable variant?
When attaching a suffix like -ness, you would normally change the 'y' to an 'i' as you have stated. However, in this particular case, "readonly" is a technical term with a specific meaning. The intent of leaving the y in this case is not because of any sort of grammar rule or variant, but instead to preserve the entire "readonly" term to make it more obvious that the writer is referring to the exact "readonly" property as opposed to a more abstract "read-only" property.
Sometimes I will write such a word as "readonly-ness" to emphasize that sort of separation of the original adjective vs the -ness suffix.
*My source is myself; I have written comments using words like "readonlyness" before for this reason