Customised js.browser_update message ignored

I updated this message some time ago but only recently have tried my forum on an unsupported browser. (I might have tried in the past and thought it was working now, as the problem rings a bell.)

Anyway, the default “Unfortunately, your browser is unsupported…” message still is displayed, not the customised text.

This is why it rang a bell:

What is the purpose of being able to customise the js.browser_update message?