The first example of encounters with extraterrestrial life stated in BT books appears to be with the Magellan Program.
As described in DropShips & JumpShips, the Magellan Program launched a series of slower-than-light unmanned probes beginning in 2029AD, with cruising speeds of 68% of light-speed. They carried a variety of sub-probes and landers to explore the destination systems.
Three of the probes (Magellan One, Four and Five) found habitable planets at Tau Ceti (later name New Earth after the Deimos Project sent humans there), Epsilon Eridani, and Epsilon Indi. Their landers on the habitable planets would be the first canonical encounters with extraterrestrial life (plant, microbe, animal, etc.)
The first face-to-face encounter by humans is, per pg137 JHS:Terra, "Engineer Michelle Land of the Terran Alliance member-state “Canada†became the first human to set foot on a world outside the Terran system on 15 December 2108, when she stepped out of one of Pathfinder’s excursion vehicles onto the world that would soon be o cially named “New Earth.â€"
Now, there are some unwritten opportunities for earlier encounters with alien life:
1) Humans might've found microbes or simple organisms off Earth but elsewhere in the solar system, such as on Mars or under Europa's ice.
2) Humans might've built and operated very large orbital telescopes that could see traces of chlorophyll or other signs of life in other star systems before the Magellan probes arrived at their destinations (c2040-2050).
3) The Magellan probes to Alpha Centauri (now Rigil Kentaurus) and Bernard's Star might've found microbes or something; per DS&JS they arrived before the Tau Ceti probe and found no habitable planets
But the first published remote encounter with extraterrestrial life is the Magellan One lander on New Earth, and the first published face to face encounter is in the Pathfinder's mission.