Developing plugins in the Dynamics 365 Online System is different of how you develop them on-premise. The framework-restriction will be covered in this article.
The first thing you have to know is, that your plugins und used libraries have to be on .Net Framework 4.6.2. Using a higher framework for plugins will result in an error during registration. You can find a hint on this on the MS-Docs (https://docs.microsoft.com/en-us/powerapps/developer/data-platform/write-plug-in).
Plug-ins and custom workflow assemblies should use .NET Framework 4.6.2. While assemblies built using later versions should generally work, if they use any features introduced after 4.6.2 an error will occur.
Based on this, using a custom library compiled with .net 4.8 will result in an error during registration or during usage of this features in your code.
Based on some new features of version .net 4.8 like Tuple you can ship this problem in .net 4.6.2 with some nuget packages like System.ValueTuple (https://www.nuget.org/packages/System.ValueTuple/).