Quantum Exactly, it's under Settings → Notifications → Notification cooldown → Use notification cooldown, which can be set to off.
It's a new Android feature added earlier this year, likely to kick in if for example you're updating apps, which generates a lot of individual notifications. Or possibly with notifications from another profile.
Personally I think it's a solution in search of a problem: the proper way to handle excess notifications is to disable those that are irrelevant and/or deal with the apps that abuse them. Nothing to be worried about though, as it's a system feature.