The command Replace with content effects that the currently set variable value is taken permanently (see under Equation or Table).

That means, that the respective variable is not used in the part anymore.

When clicking on , a confirmation prompt is displayed.

Confirmation prompt if the variable shall really be removed from the entire part.

When confirming by clicking OK, the disabled variable is highlighted by a star icon in the list. (That's the usual highlighting for not used variables.)

Variable highlighted with star icon

Also In the Sketch history, the replacement by a fixed value can be recognized.