Don't you think the customer experience is strictly more important than the developer experience?
The developer is putting in the hard yards to make the website for their community event. Why shouldn't the user make the effort to have a web browser that's updated to 2022?
It's a complex conversation, but "the customer is always right" is not always the case.
The developers do have access to statistic about exactly how many clients they'll break. They might have felt that it's so few that the ease of development justifed breaking the website for just a few users. They could tell them that their browser is too old, saving many a lot of headache.
In this case, the problem, making a reservation to volunteer, is simple enough that you could have solved it with a CGI script in 2001, and it would still work. So it only broken, because someone felt it needed to be more modern. Remember, this is people who volunteer their time, they're not forced to do anything, so you have to be as accommodating as possible.
This is a common thing when working on library-like-things outside the web too. Widely used C++ libraries generally aren't taking hard dependencies on C++20 features right now without continuing to support e.g. C++11/14/17.