No. It’s not the duty (and not the right) of web software to make users to do anything. From the perspective of Discourse, the announcement should not change anything (apart from maybe dropping some specific hacks to make IE9 display the site properly if you think the baggage is too big – but, let’s face it, there isn’t much IE-specific code left for IE>=9).
If a site owner decides to educate the visitors to his/her site, s/he may put up an information message/bar/whatever on the site to make people aware of the issue. S/He should not prevent people from accessing a site. Sometimes, people are not able to control their environment, e.g. corporate IT may decide which browser (and version) you can run. Don’t be a bad host.
If you think supporting older browser hampers development and usage of new browser features, “progressive enhancement” is the keyword. Nobody says the site has to work in any browser (and version) the same way (good luck with that), however it should work reasonably well.