NewtonSoft JSON (an open source library included in official sample projects until Microsoft made their own) was first released in 2006, AFAIK.

gp is probably thinking of system.text.json which was only added to .net core in version 3, which less than a year old. But it's not been high priority to add because of a very well established open source project called JSON.net that has been around for ages.

However the full framework has had a different JSON serialiser (DataContractJsonSerialiser) since v3.5 in 2007.

Early when JSON was new, people were using the JavaScript parser included in .NET to get the values. Then Microsoft build a small JSON parser for their ASP.NET library. But then James Newton-King build JSON.NET and Microsoft eventually switched a lot of their libraries over to it because it was just very good. It could do a lot of stuff very fast with a nice API.

Last year Microsoft introduced a new and more efficient way to allocate memory natively into .NET Core 2.0 (Span<T>). Since JSON.NET is still used by .NET Framework that doesn't support the new API, Microsoft created System.Text.Json that includes a new JSON parser that utilizes it. The first Microsoft project to use the new parser by default was ASP.NET Core 3.0.

He probably means in the standard library? According to git NewtonSoft.JSON (the defacto standard) has been around since the 2007.