Eventually after the war they all had to go home. No one really had a cause to stay together without Shepard around, there was a silent sign of respect between all of them the entire crew had mixed feelings but Tali respected Shepard for the sacrifice they made for all of them. It was unbelievable that they managed to synthesize with the reapers. ... And that it was Shepard who made it possible. But Shepard was like that, the glue of the crew, and not just the crew anymore, the entire galaxy. None of them wanted to admit it but without Shepard and the Reapers none of them had a reason to stay on the Normandy. It was just avoiding responsibility to stay, So everyone left eventually, they all said they'd stay in contact and meet up every year.
Tali as an admiral has a duty to rebuild her home, and Garrus would have avoided his duty if he wasn't worried sick about his family. After that one intimate moment they had established a relationship and talked over the extranet, eventually because Shepard was no longer around to rally everyone together they started visiting each other less.. eventually they started loosing things in common, eventually it was only politics.. and soon after that their political views differentiated. Every so often Garrus thinks back to their fight over Shepard's choice. He regretted yelling at Tali, he regretted doubting Shepard's final choice at all. Her voice rang in his head but he couldn't even grasp exactly what she said, just the rage in her tone and utter disappointment, that he had the nerve to shit over Shepard's grave by even saying it was a bad decision. He thought about those times with regret and disappointment in himself every so often. His views changed a lot over his life. But never in time for the relationships he was in. After that neither of them seemed to think they could pick their relationship up again, eventually they just stopped talking all together. And after time they simply never saw each other again. Sometimes they'd think about it. How simple it was just to completely loose someone they knew so well and even loved at one point completely. Once or twice one would search the extranet for the other and consider reconnecting. It never happened. They simply stopped they were just a part of each other's younger selves now, and that end was enough for both of them.