When playing Baldur's Gate which runs on 2nd edition AD&D rules, I found out that in older editions high constitution supposedly provided minor HP regeneration for the character. Does this really work this way and if so, does it work in 4e and 5e?
1. Do Characters in 2e regenerate hit points for extremely high constitutions: Yes.
According to the AD&D Player's Handbook on Page 15, starting at a constitution of 20, you regenerate health every 1/6 turns. Increasing to 1/5 at 21, 1/4 at 22, and so on until every turn at 25.
2. Is this present in 4e / 5e: No
Regenerating hit points every round is not something that your constitution would affect, but it does affect how quickly you heal at various points. In 4e, healing surges add your constitution modifier, and in 5e, hit dice add your constitution modifier. But, this is a far cry from round by round regeneration. That typically comes only from magic for player characters, or magical monsters such as trolls.