You can download all the headers but you can't leave the window for that group open because all the headers you download will get stored in RAM.
If you start the download then close the window or right click "Download all Headers" all the headers can be downloaded without running out of RAM.
That said, the very largest groups aren't that usable with headers anymore. What's getting posted to them isn't usable through headers, only through NZB files.
The group is a huge one that newsbin search doesn't index so even if I wanted to subscribe it won't help.
We catalog all the large groups. The problem is that what's posted isn't usable because all the files are posted both obscured and encrypted. I think you'll find even if you download the headers there's not much recognizable anymore.
The smaller more focused groups still have content usable by headers, I mean music, book, anime groups and the like.
Many people use "Sonarr" with Newsbin to download content that used to be clear text in the groups.