A company CAN extend your probationary period if they wish,and if they feel that you have not reached their expectations for the position within a given period.
However,if you have been on probation for (say) four 6 month periods and not achieved satisfactory performance then the company can dismiss you.This is unusual as normally a company would know by at the most 1 year whether you are right for them or not.
You say " forever" which has no real meaning,as we cannot define forever.IF you were with a company that keeps ectending your probationary period beyond a year or so I would leave,as they are just taking advantage of you, knowing that they can dismiss you without any comeback on your part.
You cannot argue that your probationary period has ended by default as only the company can decide when that is.It doesn't just lapse.