dashboard appears broken and looks different

Perhaps. Whilst you delayed updating WordPress, your site may have been hacked.

No, absolutely not. I have excellent security.
But let’s assume that’s the case, how would I know for sure?

Site url?

Unfortunately I cannot provide url because even if I shortenit it’s going to be here eternally.
Anyway, what will you be able to tell by seeing the front?

I have confirmed that it wasn’t hacked.

I’ve disabled all plugins and reverted to default theme.
I notice when re-enabling plugins it redirects to page not found.
This means it is now impossible to turn on plugins one-by-one to find the broken one.

I’ve also replaced the wp-config, wp-includes and wp-admin with ones from most recent backup.
Should I just use a really fresh one instead of trying to retrieve from backup?

Thanks esmi for advice.

Edit: successfully turned on all plugins with bulk action.
This means the front is looking fine – it always did, even while dashboard is broken.
It also means finding the bad plugin is more difficult (if it is a plugin).

My reason for postponing the update in the 1st place is complex.
I’m more that willing to update now but it doesn’t feel right to update something that’s not working properly…

Don’t know what to do!

Everyone: mine turned out to be related to an htacess line I copied from my guru to fight script injections.