You can safely leave during draft or before creeps spawn and you will not lose MMR, behaviour score, get low prio or nothing. The only penalty is 30min queue restriction, and if you do it too often (every 15 games) it WILL count as a loss and a proper abandon.

At immortal draft it is 100% needed since there are people that hate eachother and if you draft them together they will spend 20 minutes feeding down mid or stay afk in trees near the mid t1 tower until the game is over. Watch any high mmr streamer and even they do it from time to time becauae you rather chill foe 30min and do somethijg else instead of spending 20 minutes waiting for the enemy team to take down throne.