Concretely, you answered your own question. The "star" button is the default way of saying "thanks" to a project which was useful to you.
> is it really considered so unusual for a someone to express gratitude to an open-source project?
I would say yes, it's very unusual. The general lack of basic civility is, in my opinion, just one more reason for the lack of diversity in projects and lowers the number of contributors to open source.
Not exactly, as star numbers are made public, meaning it's also a form of public approval.
In the olden times, we all just used email all the time for everything. It would've been bizarre for someone to work in open source and not have their email publically available. The funny thing is, I haven't shared my email on my GitHub either! I'm going to fix that today.