thisThis is entirely relative...aka aka, it all depends.
i've I've never had a case where iI decided to give up on an ieIE version...are Are you building with web standards? and testing? what's the major issue(s)?
a few thoughtsA few thoughts:
no
No it is never acceptable to shut down browsers and shut out users. neverNever do that.
ok Ok so you have to do it...your Your plan to ice out < ie9IE9 via cc is correct. do that*Do that.
*: don't Don't just serve up display:none..display:none
.give Give the user an explanation/context/linkexplanation/context/link
to newer versions, other browsers browsers, etc.
segue rant:
youSegue rant:
You are worried about best practices, so iI have to point out that employing the best technical practices along with uxUX/uiUI practices is the way to go. servingServing up nothing to a particular user is the worst thing you can do.