Top 20 NuGet placeholders Packages

Configuration provider for resolving property placeholders in configuration values
Packing for adding property placeholder resolving config provider to ASP.NET Core applications
Simple providers to generate pages with basic replacement mechanisms
Holder uses the canvas element and the data URI scheme to render image placeholders entirely in browser.
A string composition library written in C# that can format data into a string with a minimal, intuitive syntax. It uses extensions to provide named placeholders, pluralization, gender conjugation, and time and list formatting. Project hosted on GitHub https://github.com/scottrippey/SmartFormat.NET ...
An implementation of IVariableSource of Spring.Core where you can declare your variables/placeholders in a validatable json-file.
Use this library to replace placeholders in a template string.
Placeholder support for the ASP.NET Core Configuration framework.