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Dahomey.Json
The main purpose of this library is to bring missing features to the official .Net namespace System.Text.Json
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I'd add that there are some libs that enables features of Newtonsoft.Json for STJ. E.g. this one is pretty exhaustive.
But TBH the only feature I really missed when I work with STJ is polymorphism support. You don't even need a 3rd party lib for that: it requires some gymnastics but it can be implemented relatively easily by creating a custom JsonConverterFactory + JsonConverter. This implementation of mine can give you some ideas. (Also does type whitelisting based on Protobuf configuration.)
That's we've got currently. Things may improve in the future. Luckily, you always have Newtonsoft if STJ is not sufficient.