Exactly, this is another unexpected consequence of Brexit. The EU has recommended that citizens of the EU and EEA as well as UK to be permitted free movement, but that isn't binding on all members, hence individual member states were free to require quarantine, even to ban people from entering their countries. Most nations have however not gone to such extreme measures. After 1st January the rules will be different for UK, and it is entirely plausible that countries, such as, say France could refuse admission to France from UK for tourism, unless for example they had got a negative PCR test, or a vaccination certificate, which the UK is so far refusing to issue.
All this has nothing whatsoever to do with trade agreements, and isn't "project fear", it's a bi-product of leaving the EU, and absolutely nobody knows how much it may negatively impact the leisure travel industry into 2021. Still, there's always Turkey!