You are correct that Increment Value and Run Section tool has been removed in R2021b, checkout release Notes
To increment a numeric value within a section, use controls in the Live Editor. For example, this code calculates the factorial of the variable x.
To interactively change the value of x, in a live script, replace the value 5 with a numeric slider. By default, MATLAB reruns the current section when the value of the slider changes.