Just got hit by this little surprise myself, accessed notes via web and got the message to upgrade, said no thanks and closed browser, didn't actually access any notes in the web.
This morning, open my desktop and it said I needed to de-sync a device before I could access any notes. Desynced the unused web client and continued.
Oh and you can only de-sync 4 times a month. If I were (extremely) forgetful I could lock one of the devices I actually use out of my notes. Yikes. Things must be getting tough up in Redwood City.