Till has been in use in English since the 9th century; the earliest sense of the word was the same as the preposition to. It has been used as a conjunction meaning “until” since the 12th century. Until has been in use as both a preposition and a conjunction for almost as long. Both of these words are acceptable; you may send a text to your misbehaving child stating either “U R grounded till 4ever” or “U R grounded until 4ever.”
… you will probably wish to avoid ’till, use ’tiladvisedly, and use both until and tillfreely. And if you use till in writing and someone tells you that you have made an error, simply take the extra L off the end of the word and poke them in the eye with it.