In general a lot has changed for the better since 4.x and mono.

> Though, I chuckle at the idea of companies upgrading such to errors via config-

Depends on the company's code quality standards, of course. A lot of companies in my experience let tools like SonarQube drive towards very strict "linter" configs.

I also almost failed to mention it (but did in a quick edit), but I also think that the cross-platform compatibility issues in this case also especially drive companies to avoid BinaryFormatter "naturally". The .NET team made a series of correct decisions that while they couldn't break backward compatibility (all the way back to the OG .NET Framework 1.0) they could keep cross-machine compatibility broken (it was always broken, unlike Java's Serializable, BinaryFormatter was always primarily for FFI and local single machine use, which anyone trying to use it for distributed serialization almost always found out the hard way eventually) and even expand it to be "more obviously broken". One of the core abilities in .NET 5+ is to be able to build/test locally in Windows or macOS and then push to, say, Docker containers on a Linux server. BinaryFormatter absolutely does not work in such scenarios and makes a loud racket if you try. Additionally, there are even now subtle incompatibilities between say type of Windows machine.

Getting bit by that is easy, and will also naturally cause companies to expedite moving warnings to errors in their configs.

On the carrot side, too, System.Text.Json is now out of the box in every .NET allowing for very easy (and quick/performant) JSON serialization. Not having to install a third/second-party library for a secure, standards-based serializer is a big deal and helps remove a lot of the reasons people would accidentally rely on BinaryFormatter. (Similar to the old days when Python only provided pickling out of the box and it took maybe too long for a json library to move to first party standard library inclusion. It's great that both languages have solved that.)

Yea, .Net has massively improved. I just had an awful employer who burned me out- possibly for good.

I don't think I need to say more than: 98% of the code base was ASP.net backed by .Net FW 1.1, the rest was VB6.