Over the past four or five years, games had tended to go down to the last few minutes with only a few points separating the sides.
With the Rugby Championship decided a week earlier, the 2021 champions emerged from the series with plenty to ponder.
Foster said: "We've got a tight group that's excited about growing. We approached these last two Tests as a chance to test ourselves against a foe we know plays a style that can suffocate you, and we got some lessons out of that.
"We showed we can deal with that, but we also showed we need to deal with it for longer periods," he said.
That included the final moments after fullback Jordie Barrett landed two crucial penalty goals. The All Blacks managed to regain control of the ball from the final restart only to infringe at the breakdown. That allowed South Africa to kick to the corner. They employed their lineout maul to suck the All Blacks into an offside penalty in front of their posts.
"We are bitterly disappointed. It's frustrating when you've got control of it, but it was a massive arm-wrestle. The South Africans played superbly. They came with a lot of attitude; they carried hard, they moved us around, and challenged us in many ways," Foster said.
"They had a strong third quarter and got us a bit flustered.
"I loved the way we hung in there. We took some great opportunities early in the game and got ourselves back into a winning position but then lacked a little bit of discipline in that last two minutes.
"That's what big Tests are about.
"It just shows we've got a little bit of learning to do, but I'm intensely proud of a campaign that we've taken one big step forward, but we've got a few more to take," he said.
The All Blacks had grown considerably, and he was proud of how the players handled the last five to six weeks.
In the final quarter, the All Blacks had struggled to make an impact with the ball in South Africa's half.
"That was the area I thought we got frustrated and didn't have quite the direction we needed so we'll got and have a look at that," he said.
All Blacks captain Ardie Savea, who did much in the final moments at breakdowns to secure what could have been a winning advantage, said he had issues with referee Matthew Carley's interpretation of the breakdown. That was especially with South Africa not being hit for their inability to roll away after making tackles.
Savea said, "I have my own opinion, but it's the referees' call at the end of the day. I wish I could go back and change the calls, but he made his decisions, and we just had to adapt.
"We were well aware around the breakdown of staying on our feet, but we didn't execute that skill in the last couple of minutes.
"We've got to take the referee out of the game and we didn't," he said.
The All Blacks had not reached the point they wanted to be playing. South Africa put them under pressure to highlight the areas New Zealand needed to improve, he said.
They now have two weeks to prepare in Australia before heading to their northern hemisphere.