Why are the square root symbols on this post different? Some of them have vertical bar and other ones have slanted bars.
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That is MathJax rendering not TeX so not really on topic for this site however you would see similar from a TeX rendering.
TeX fonts have a small fixed set of square root signs (typically with slanted left bar) but then once the term reaches a certain size, TeX switches to a use a glyph for the lower part and then repeats as many "extension pieces" of a small vertical segment as needed. This last mode necessarily has a vertical left side.