If you're working with C#, Microsoft added first class support for feature management in .Net with https://github.com/microsoft/featuremanagement-dotnet. It provides the logic for simple on/off, user targeting, percentage roll outs, and schedules. Using feature management along with flags hardcoded in `appSettings.json` makes perfect sense for a simpler application.
When your architecture grows and you start needing to synchronize configuration across multiple applications, or have non-technical users change flags, then you've probably outgrown hardcoding and need to consider building (or buying) a service. You probably don't want product owners trying to set an env var like `FeatureManagement__NewFeature__EnabledFor__0__Name`. Azure App Configuration is the go to if you're willing to use Azure - it provides a nice UI for editing flags instead of dealing with JSON. Or, I've been building https://featureflags.app/ as an alternative provider if you prefer to steer clear of Azure. Its kinda the middle ground between hard coding and a full on platform like LaunchDarkly.
There are many options in this space. But if you don't want to roll your own provider, the only way to use Microsoft's management solution is via Azure. Or try to manage it all via json files - which potentially require infrastructure to manage if you have a more complex architecture. Some developers prefer to avoid using Azure, and this provides the infrastructure they need.
This way of thinking is absolutely still around. I think AI may actually be making it worse. Now managers have it to validate their ideas and they don't think they have to listen to feedback from subordinates because some AI agrees with them.
I've spent a few months building a simple feature management system for .NET - https://featureflags.app. The .NET built in feature management libraries work pretty well. But I wanted an easy to use UI for configuring flags - without having to use Azure.
When your architecture grows and you start needing to synchronize configuration across multiple applications, or have non-technical users change flags, then you've probably outgrown hardcoding and need to consider building (or buying) a service. You probably don't want product owners trying to set an env var like `FeatureManagement__NewFeature__EnabledFor__0__Name`. Azure App Configuration is the go to if you're willing to use Azure - it provides a nice UI for editing flags instead of dealing with JSON. Or, I've been building https://featureflags.app/ as an alternative provider if you prefer to steer clear of Azure. Its kinda the middle ground between hard coding and a full on platform like LaunchDarkly.