The "Travel Abroad/Foreign Travel Advice" section of the FCO website is annoyingly organized by country - so for alerts about Europe, the user has to navigate country-by-country.

To get this piece of basic advice, you have to pick a specific country:

"Summary – there may be increased security in place over the Christmas and New Year period, including at Christmas markets and other major events that might attract large crowds; you should remain vigilant and follow the advice of local authorities."

That's from the Austria page, and is likely repeated for all or most other countries in continental Europe.

It looks like all the country pages for Europe were updated like this on 21 November.