C# represents about 12% of the jobs out there.
https://www.devjobsscanner.com/blog/top-8-most-demanded-programming-languages/
It's not unpopular, but it's definitely not "massively" popular anymore. Different languages have different strengths and weaknesses, but C# has a few more weaknesses than most. In my opinion. Yes, there's nothing wrong with learning any particular language you want to learn (and I'm a little surprised to see C++ has fallen significantly lower than C#, sure, fair enough). I'm just struggling to see an upside for learning it in the modern day (and now knowing more about what this person's goal is I feel even more strongly that C# is the wrong answer for them. In my opinion.)
IDK, I just have never really had this become a serious issue for me. I get what you mean, some actions are a little bit of a pain in the neck because people are often sloppy about typing, but literally the only time I can remember it being an issue at all has been when numpy is involved and so I have to figure out if something is a native Python thing or a numpy-fied custom structure.
I mean there's just not that many types. Generally something is a list, a number, a map, or a string, and it's pretty obvious which. Maybe there are OOP domain things where a lot of variables are objects of some kind of class (sort of more of a C++ type of program structure), and so it starts to become really critical to have strong type tools, I'm just saying I haven't really encountered too much trouble with it. I'm not saying it's imaginary, you may be right in your experience, I'm just saying I've worked on projects way bigger than a few hundred lines and never really had too much of an issue with it in practice in my experience.