It seems so: https://plus.google.com/u/0/downgrade/
I get pestered every time I log in.
I have a standard Google Account username (i.e. my username and not my real name appears when I comment) and it's not linked in any way to G+ (although I _do_ use G+ often, might that be the difference?).
https://support.google.com/hangouts/answer/2944865?hl=en&ref...
I know for a fact it is impossible to use Hangouts on iOS devices without maintaining a Google+ profile. A friend was forcibly signed out when she deleted the profile it created for her using the iOS app. Either Google+ is on or Hangouts isn't a stable option.
Edit: To clarify, two separate friends who don't like/use Google+. Sorry if I'm being confusing when trying to add context.