Posted: Fri Aug 21, 2020 8:41pm
Firstly, 90 days in 180 won’t start until 31/12 and freedom of movement ends for UK nationals.
But to answer your question, once Transition period ends and UK citizens become Third Country Nationals (TCNs) what stops you is that 90 in 180 refers to the entire Schengen Zone, not the individual country. So even if you spent 88 days in Spain, then spent three days driving back to UK through France, you would be found in breach of the Schengen 90 in 180 for TCNs rule at the French border and likely fined and possibly banned from re-entry to the EU / Schengen area for up to 5 years.
Once transition period ends, there will most definitely be more attention paid to UK citizens time in Schengen Zone and if they have overstayed by any significant period. 1 or 2 days MIGHT not be an issue, dependent on the country you leave from - but a week or so will start causing issues.
When ETIAS comes online in 2023 that will be the death-knell for casual overstaying and getting away with it.