if you have a 90+ character doing normal baal runs with level 20-40 characters, does it negatively impact the experience gained by the lower level party members?

Yes.

yup. thats why there are enchantresses on norm-nm to help low lvls lv up on their own

Not allowed to post links on this forum but the Arreat Summit site has details around how the XP is shared in parties, bonus for being in parties, bonus for being in players 8 and penalties.

Short answer yes, the higher the char level, the higher the share of the XP pool they get.

yes, the experience is shared and weighted by the individual character level

https://classic.battle.net/diablo2exp/basics/experience.shtml

Which part was forbidden ?