A discussion is ongoing about customizing the “outdated browser” message on Discourse forums. fysics suggests making the message editable from /admin/site_contents, rather than requiring edits to client.en.yml. Proposed customization options include changing the link to a different site (e.g., whatbrowser.org) and adding the community name to the message. riking notes that plugin developers can override specific strings in client.en.yml by including a locale file in their plugin. Meanwhile, codinghorror has shortened the default message to make it more concise.
It can be adjusted by editing client.en.yml as described above. However, I think it would be beneficial for it to be adjustable from /admin/site_contents.