Having to use the “Use as mask” option again to release a mask isn’t the most intuitive solution (hence me ending up here to find it out, rather than this being clear in the UI).
It would be great if Figma could detect you’ve got a masked object selected and change the option text to “Release mask” accordingly. Or, to be consistent with the way normal object Grouping/Ungrouping works, perhaps it would be better to always include both “Use as mask” and “Release mask” options in the menu, with whichever one is not applicable to current selection being greyed out/disabled.
At the very least it would be helpful if the current solution could be documented in the Figma help articles on Masks! (example: https://help.figma.com/hc/en-us/articles/360040450253-Masks).