I recall on previous versions of IPB a blue (or sometimes red) banner / notification would appear when logged into the board as administrator and a software update was available. I don't seem to get those banners anymore. Were they removed or am I missing something?

These still exist if you are logged in as an admin who has the permissions to upgrade, and your admin CP is showing the upgrade available (by this, I mean an upgrade check has been done). Given you have upgraded, its very likely it had simply not run yet

I just upgraded - it was two versions out of date according to "Get Support" in the admin CP.

Please let us know if you are not seeing this on the next release, but please do not upgrade. Also, please check on a default unaltered theme at that time. Of course, at present we would not be able to investigate this as you are up to date, so there isnt anything for us to see there.

I'll take a look for next upgrade.