Per Gerhard Schroeder, Germany’s former chancellor

According to Schroeder the deal would have included the following main aspects:

· Ukraine would abandon its Nato aspirations;

· The bans on the Russian language in Ukraine would be removed;

· Donbass would remain in Ukraine but as an autonomous region (Schroeder: "Like South Tyrol");

· The United Nations Security Council plus Germany should offer and supervise the security agreements; and

· The Crimea problem would be addressed.

“Umerov opened the conversation with greetings from Zelenskiy. As a compromise for Ukraine's security guarantees, the Austrian model or the 5+1 model was proposed. Umerov thought that was a good thing,” said Schroeder. “He also showed willingness on the other points. He also said that Ukraine does not want Nato membership. He also said that Ukraine wants to reintroduce Russian in the Donbass. But in the end, nothing happened.”

“My impression was that nothing could happen, because everything else was decided in Washington,” said Schroeder, who had two sessions of talks with Umerov, then a one-on-one meeting with Putin in Moscow, and then a meeting with Putin's envoy.

Lots more detail here: Source

I don't think it's ever actually been confirmed what the terms were. Possibly including giving away Donetsk, Luhansk, or both?