Are people using this file format a lot in the wild? I look at textbundle.org and it looks like a lot of respected markdown processors and editors are using it. I'm just curious as to if people are using it as a common format to exchange documents between these editors.

I don't, but I would if programs I used supported it. Textpack too.
afaics it mostly exists within the Mac ecosystem and doesn't have enough support for effective lift off.
But including images within the file is a far more robust system long term than relying on links remaining active.

Textbundle was created over a decade ago as a collaboration between Brett Terpstra and the devs at Ulysses to make a format that could be used to move files between sandoxed Mac apps, which otherwise would require tiresome manual access, individually, to all attachments in a file.

https://brettterpstra.com/2014/08/26/announcing-the-textbundle-format-and-ulysses-3-giveaway/

https://blog.ulysses.app/introducing-textbundle/

I own and use several of the apps listed on the Textbundle site, but I suppose I don't have many asset-filled files I've needed to share, as I don't recall ever exporting/importing to that file format. I don't have much need to move files with images in them into other apps, and when I need to export I typically make PDFs.

I could see the format being useful for turning asset-filled docs into blog posts, so the integration of the Textbundle format into the iOS/iPadOS versions of the Wordpress app could be useful.

There are only two apps supporting the format that do not have a Mac version, and not knowing anything about Windows security I don't know if the format actually solves a problem on that platform. (One of those, Smartdown, never hit v.1, never delivered a promised Mac app, and the dev apparently focused his attention on becoming the Windows dev for IA Writer... which doesn't support Textbundle).

satis wrote:

not knowing anything about Windows security I don't know if
the format actually solves a problem on that platform

I don't think textbundle solves a Windows security problem - apart maybe from removing the possibility that links might turn evil.
It does make markdown files with media more equivalent to docx. I've wasted a fair bit of time dealing with missing links. Enough for me to have stopped using markdown/Obsidian/Tangent for files with media. I subscribe to Pocket purely because that guarantees me access to the original images rather than relying on websites maintaining them.