Now that Brexit happened, can someone from the EU travel for holidays to UK with their National ID card (which has never been a problem), or now they need a passport? Even before 2021?
Although the UK has formally exited the European Union, it is in a 'transition' or 'implentation' period until 31 December 2020. During this period, the UK remains in the single market and customs union and such rules continue to apply as they did while the UK was an EU member state.
If you are in any doubt, you should contact your passport authority and carriers with which you intend to travel.