There are two separate legal things in play here.
Until 31.12.20 UK citizens are still notionally EU citizens for the the purposes of free movement within EU countries.
They are not yet in the "Third Country citizens" Schengen category.
Thus the only limit on them wandering freely and unrecorded in Spain and through borderless Schengen zone countries is the old, now totally moribund "you can only spend 90 days in our country without registering" Spanish domestic law.
That old Spanish law ceased to have any practical effect at all donkeys years ago as soon as Spain joined the borderless Schengen zone group of countries.
And ever since then the Spanish authorities have no idea how long you've spent in Spain versus Portugal, France etc.
But UK citizens cease to be notional EU citizens after 31.12.20. From 1.1.21 onwards they are Third Country citizens.
The longstanding Third Country Citizens Schengen Zone Visitor rules start to apply to them from that date.
Those existing rules say that unless you've got legal Residence status in an EU country (or an EU longer-stay lucrative visa) then you can only visit the Schengen zone for 90 days in any 180 days period.
The key thing here is that whilst the old Spanish law of "only 90 days in our country" was never applied as an overstay could never be proved (and frankly no one really cared anyway), the Schengen-wide 90 day visitor limit for Third Country citizens IS VERY MUCH applied, because it’s dead easy for the Police throughout Schengen and all border control immigration authorities to know exactly how long you've been in the zone.
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1. Your passport is machine-read at any entry point into Schengen.
2. All entry point computers are linked to the single Schengen-wide database.
So whenever you try to leave Schengen (or you are passport-checked in a police stopcheck anywhere in the Schengen zone) the system immediately tells the officer how long you've been in the zone.
More that 90 days in the last 180 days? - and bingo:
Arrest, fine on the spot and immediate deportation from the Schengen zone, plus a ban on re-entering of between 1 and 20 years depending on the severity of that criminal offence, with the norm being at least a 2 years ban. European countries have been operating this system for years with non-EU countries!
However, any start date of a Brit visiting Spain (or anywhere else in Schengen) prior to 1.1.21 doesn't matter, as Schengen Third Country citizen visiting limits didn't apply to them then.
But assuming that their entry date was before 1.1.21, the "Third Country Visitor" Schengen clock starts ticking for them on 1.1.21.
Thus, they have to have LEFT the Schengen zone by 31st March 2021 at the latest (90 days after 1.1.21).
If they are caught in Schengen after that by an internal stop-check or at any border control when attempting to leave, the Schengen immigration control computer will instantly flag them as an illegal overstayer (cos their passport has not got an entry date in the system within the last 90 days).
Bingo!
Arrest, fine, deportation and re-entry ban.
As Brits, we've never seen or even noticed this Schengen zone 90/180 rule, nor seen it being enforced, as it has never affected us. We were never Third Country citizens.
But it's been in force, and enforced, for decades against all Third Country citizens visiting Schengen.
In 2019, 142,000 Third Country citizens were deported from the Schengen zone after being caught still present in it illegally.
So, the under-the-radar Brits living in Spain have until 31st March 2021 to finally get out of Spain and Schengen. And if re-entering Schengen after that they will be subject to the strictly enforced 90/180 visitor rule on all future visits.
It's going to be interesting to watch... and hear the comments: “but we never knew, no-one told us...”
Chris
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