Texts were added, removed and edited.
That's the kind of fact-free claim atheists always say.
Manuscript evidence is the only fact-based way of determining such a claim. There's zero manuscript evidence of that in the Old Testament.
The Dead Sea scrolls discovered in 1947 provide a set of manuscripts 1,000 years older than any that were found prior to that. It contains parts of every book of the Old Testament with the exception of Esther. It incudes virtually the entire book of Isaiah. It was assumed by certain people that there would be lots of differences between the older manuscripts and the more recent ones. There were essentially no differences.