If its hard to know how many people are actually using Internet Explorer:
You could make the next release of pgAdmin display a message occasionally to users of Internet Explorer saying that Internet Explorer will no longer be officially supported in a future version, and when that version comes the message says now no longer supported.
You can then see how many people contact you about this to express concern.
Good idea. I've hacked up a patch to warn users if they're using a deprecated or unsupported browser.
