Pretty rare these days though.
Welcome to Python 2 / Python 3 version of .NET evolution.
There are a lot of libs that haven't been updated for years and therefore don't support netstandard. But you probably don't want to be using them outside of very specific use cases given they are likely to be insecure by now.
There definitely isn't a 'we're holding on to netframework' vibe in dotnet, like there was for python2.
To the point that some companies rather spend their budgets into full rewrites into other stocks, or acquisitions, instead of porting into .NET.
See Sitecore, as an example of what was once a key player in .NET CMS enterprise space, now focusing into other stacks.
https://www.sitecore.com/landing/uk/2023/the-future-is-compo...
Most of the new acquisitions are based on Java, with exception of Order Cloud and Content Hub.
.NET Core is available for XP headless rendering SDK, however it is NextJS/JSS that are getting the spotlight and there is plenty of DIY if you will go down .NET Core, specially after the cooperation agreement with Vercel.
Classical XP is stuck on .NET Framework and XM Cloud customisation points favours WebHooks instead of writing .NET code.
CDP, Search, Send are low code tooling, only exposing Web APIs, and JavaScript.
Content Hub does allow for some use of C# in scripting, but it is mostly about Webhooks as well.
https://doc.sitecore.com/xp/en/developers/hd/210/sitecore-he...
However as you can see, XM Cloud clearly favours JavaScript, specially in the components that replace content editor and page graphical editing,
https://doc.sitecore.com/xmc/en/developers/xm-cloud/sitecore...
And then check the acquisitions, https://doc.sitecore.com/