I have put in performance based vesting of restricted stock units, straight stock grants, or stock options, but these equity grants typically need board approval or at least put into a board approved stock option or equity inventive plan. That is sometimes difficult to draft into a grant since determining if someone hit those performance metrics is not always black and white. The board and management are typically constrained by the terms of any approved plan, state and federal securities laws, and considering what others in the company will say if one person gets substantially more than someone else.
To answer your question, the typical equity or stock option incentive plans set aside a pool of a certain number of shares and when one person leaves, they usually have a short period of time (often 30 to 90 days post termination) to exercise any option rights they had. If they don't exercise it, they lose it and that grants goes back into the pool for other employees. You can ask for performance reviews for possible further equity or option grants if you hit certain performance targets every year, quarter, etc. That is pretty fair to both the company and employee most of the time.