You might find the following update helpful. This comes from very reliable contacts from outside 41 Club.
“I felt it important to write to you to highlight a particular issue that is arising for some of our people, those who would probably understand themselves as ‘swallows’. By this I mean those who perhaps have a holiday home in this part of Europe and come out here for any length of time on what is seen as a temporary basis.
We have been told of people who perhaps have been here for some time, since before the New Year at least, maybe because of travel restrictions due to CV19 or for more immediately personal reasons. It seems that if they have been here continuously since the beginning of 2021, the 31st March represents the 90 day limit that is now permitted given that any of us may be UK citizens. This will apply to people who are not in possession of a residence card or equivalent.
There is the risk that after the 31st March a €3,000 fine can be applied to a person who overstays. In addition, when such a person does then leave, their passport will be marked with a stamp saying that they are an ‘over-stayer’ and that may mean that they are then refused entry again for the following five years.”
This is in line with the agreement entered into by the UK government.