wasnt that done to stop people from sort of mmr bugging? .. 3 people gain a ton of MMR on one team then jump to another team with crazy MMR and sky rocket passed others ..
Nothing like that has ever been implemented.
In s9 and before a team's MMR was the average sum of 2 / 3 / 5 players' individual MMR (and a player's MMR only changed on an individual level), hence why playing with 2 guys on 1k personal MMR and 1 main was so rewarding. (the main still gained rating, but assuming that he had 2.4k individual rating his team was only playing at a grand total of ~1467 MMR, therefore meeting teams way below their skill level. keep in mind that in this system the highest guy only gained from this as all of his wins still contributed towards his own MMR, despite the team being far lower than he himself was/is)
From s10 and forth a team's MMR is still calculated the same way, but the overall MMR is evenly distributed between every team member. In this instance if 2 players on 1k individual rating team up with a player from 2.4k rating all of their individual MMR is going to change accordingly, meaning that the highest guy is going to be dragged down, while the 1k rated players are going to gain slightly. (in this scenario all of them would end up with an individual rating of ~1467, which would at the end of the day add up to the exact same teamrating)
Your team does not suffer as long as you don't try to play with players far below you. This system does punish players that teamhop very often, but I doubt there is a better way to deal with this problem. (and it's not an immediate -200 MMR or whatever else the legend might say)