I don't really agree with this. Many projects enjoy a vibrant development community on mailing lists today. Cairo suffers from different issues - and ultimately in Cairo's case it doesn't really matter. Cairo has been "done" for years, it's not the end of the world if it doesn't see much activity.
>Do people really want to deal with text patches in 2020?
Yes. You'll find many of them in this very thread. SourceHut is a brand-new service, only around for 2 years in alpha, and in 2020 alone it transacted 37,000 emails. There's obviously demand here.
One thing that can matter a lot if you're trying out the email workflow is making sure you're using a good client. I made one that I recommend as such: https://aerc-mail.org. Other good choices include mutt and alpine.