Steam

The tl;dr seems to be:

After such and such date (end of the month I think? Sometime soon either way) you will be required to sign in with a PSN account in order to play. Some say this was not advertised when the game launched, some say it was (I believe people found wayback machine captures that say it was but I may be wrong. Some people don't want this, some countries aren't allowed to sign up for PSN accounts outright (and if they have Helldivers 2 they would then be losing access technically).

Sony offered up the usual explanation of security and whatnot, which most aren't accepting that as Valve/VAC do the job of security, leading most to assume it's a way for Sony to gather more info and data from customers which they can then sell or otherwise market somehow.

Overall souring people on the whole situation of a bit of a bait-and-switch.