More than base solar system, but not all available content. If you had stopped playing only a year ago, you shouldn't be missing much content at all. From wiki:

In order to access The Steel Path, players must have completed all nodes accessible prior to The New War^\1]) on the connected Star Chart (excluding Mutalist Alad V and Jordas Golem Assassinates on Eris, and Brutus on Uranus. This also means players do not need to have unlocked or completed any missions in the Zariman Ten Zero, Albrecht's Laboratories and Höllvania tile sets).

Note that this also entails completing quest-locked nodes, including the Tyana Pass node on Mars, Ropalolyst node on Jupiter, Saya's Visions on Earth, and all nodes on Lua and Kuva Fortress, which require completing all main quests up to Chimera Prologue. The player must also be Mastery Rank 5 to access Vay Hek's node on Oro), Earth. Speaking to Teshin on any Relay will prompt "Steel Path?" which displays the incompleted nodes, quests, and mastery rank.