For those who have Nelnet as a servicer, here’s the message that shows when I just logged in:
So it looks like (for Nelnet in particular, at least) under some circumstances (i.e. if your account is overdue) they’ll automatically apply it, but otherwise you might have to request it. This link has more details re: forbearance, capitalization, and the temporary 0% interest rate.
I believe Mohela is also applying it automatically for overdue accounts, but otherwise you have to request it.