There is no “previous”
\ell. That glyph is the one in Computer Modern Math and the shape is what the font designer (D. Knuth) decided. The designer of STIX Two chose a different rendering to blend it with the other lowercase calligraphic (cursive) letters, of which the font has the complete set.
To the contrary, Computer Modern has just the “cursive l” in that set.
This said, you can import the
\ell of Computer Modern in quite an easy way. In
fontmath.ltx you find
so all it takes is
This is $\ell$. Note that it's different