Edit. I was not aware that the gap between System.Text.Json and Newtonsoft narrowed, take my comment with a grain of salt, please!
Newtonsoft.Json as the primary JSON serializer (at least in every place I've worked) has NOT been the case versus System.Text.Json for years. Though it certainly used to be the case.
Because there's a boatload of older .NET apps that have been using Newtonsoft for over a decade already and aren't in a rush to switch. Anything built on .NET Framework is likely to still use Newtonsoft.
Those older stuff are likely running newtonsoft.Json anyway.
There's also a load of .NET Framework applications still running Newstonsoft.
I haven't tried it because it has generally seemed easiest to use the new attributes. Though a large part of that is the shift from the WCF-era "opt-in" approach of DataContract/DataMember [0] versus the "opt-out" JsonIgnore approach where most of the transition is deleting lines because JsonIgnore is the exception rather than the rule. You could even keep DataContract/DataMember during the transition and just take an additive approach for JsonIgnore.
[0]: It was a good idea, in theory, but so annoying in practice.
It's especially annoying in libraries because the consumer might not even be using System.Text.Json, but you have to pull in the library anyway to apply its own attributes if you want it to work right. Just an overall awful smell given the existence of first-class framework attributes to do the same thing to avoid just that problem
Case in point: https://github.com/dotnet/efcore/pull/28084/files
That library is also out-of-the-box in the BCL in .NET > 5, though. It's not an extra install (except for projects stuck in .NET 4.x).
I get the desire to avoid that sort of redundancy in the BCL itself, but also explicit is better than implicit, which was the baseline theory for why System.Runtime.Serialization was the way it was even as it added more and more implicit "conventions", and if System.Text.Json supported the SRS attributes out-of-the-box there would be just as many complaints about "implicit magic" or it not following WCF conventions hard enough and that making it harder to migrate things using SRS attributes when you wanted different behavior in JSON serialization than you would want in WCF SOAP serialization. SRS has too much WCF baggage, unfortunately for all of us.
Perhaps what's left of the disagreement here between us is if System.Text.Json counts as "first-class framework attributes" and it certainly seems that way to me in .NET 5+, and especially as we celebrate the release of .NET 10. It's a small redundancy in the BCL, but it's still just "as BCL" and "first-class".