There are number systems where it becomes untrue, but
I tend to hear this a lot. Seems like no matter what ypu say, it's not true in some number systems. But nobody ever names these number systems.
So question, which sytems are you talking about?
There isn't one.
As has been explained multiple times over the years, there is a number that is infinitely close to 1 but isn't 1, however that number is not 0.999... That number is objectively equal to one in all systems of math which are based on the standard definitions and axioms that we do all math with today on planet earth.
If a system exists where 0.999... doesn't equal one, it doesn't exist on planet earth, and may not exist anywhere except inside the head of a few dingbats.
I suppose it technically doesn't equal 1 if you're writing a hexadecimal number. You'd want 0.FFF... for that.
tiornys was speaking of analysis-type number systems that are not used by people outside the field of mathematics and theoretical physics ---
And when the hourglass has run out, eternity asks you about only one thing: whether you have lived in despair or not.