I\'ve been keeping my head down working on various projects and apparently Microsoft has been busy making some big changes and it\'s confusing the hell out of me. ASP.NET Co
This answer will try to focus only on ASP.NET MVC 5, and as little ASP.NET Core as possible.
Product Lifecycle
Microsoft still supports this, and there is not yet an end-of-life announced,
https://www.asp.net/support
Framework/Product Retirement
ASP.NET MVC 4 July 1st, 2019
ASP.NET MVC 5
So if you like, keep using it. Remember that VB6 and classic ASP users are still being supported by Microsoft.
.NET Framework 4.5.2 and above are still supported if you do check Microsoft documentation on product lifecycle.
Maintenance
However, you do need to notice ASP.NET MVC 5 is on maintenance mode, as development resources are almost all on ASP.NET Core right now.
You do get,
If you monitor the relevant NuGet packages, you should notice that even recently Microsoft updates them to remediate security vulnerabilities and so on.
The notice you saw from Microsoft Docs, in fact emphasizes on the very first sentence "We’re no longer updating this content regularly." That makes perfectly sense as ASP.NET MVC 5 is rock solid so you should not expect new materials to be added any more.
You said "Since creating that project, I found what seemed like some bugs in both MVC and Entity Framework only to find that the only mention of them on the web seems to be in Core and that's the only place they're being considered being fixed."
Well, it really depends on what "bugs" you are talking about. Like I said earlier, security related issues are still being patched, but bugs with workarounds or functional limitation are least likely to get fixed. It is an open source project, so if you really want, you can fix the issues on your own, as last resort.
On new features side, Microsoft does backport some features from ASP.NET Core, such as dependency injection, new configuration system, to simplify migration at certain degree. But don't expect much.
Migration
Do consider migrating to ASP.NET Core if you can.