I don't think it's such a huge issue as you make it to be. The documentation states this only as an alternative: https://phpspreadsheet.readthedocs.io/en/latest/topics/accessing-cells/#looping-through-cells-using-indexes
It also mentions the pitfalls. I doubt many users would prefer that "alternative" given that the recommended way is simpler and does the same thing. I also don't think that performance would come into play here. Any difference would be insignificant. I believe that the benefits of the deprecation outweigh any potential extra work the developers may have.
