Whilst the UK was a member of the EU Spain did not enforce the 90 day rule - the rule was always that if you intended to stay longer than 90 consecutive days then you had to apply for residency. Spain's tax laws mean that (resident or not) if you stay 183+ days in a year then you must file a tax return on your world wide assets and pay due Spanish tax. These have been greatly ignored and were hard for the Spanish to police for EU nationals, so they usually didn't. With ETIAS coming online next year that will change for non-Schengen nationals anyway, so Brits would have started to be caught out next year.
The difference now is before we could stay 90 days, pop out of Spain for a weekend then return to Spain resetting the 90 day clock, so as long as the total days in a tax year stayed under 183 you were within the law. Now it is 90 days per consecutive 180, but overall the amount of time you can spend in Spain in a year has not decreased.
The 90/180 day rule is a Schengen one so I think it unlikely that Spain can make an exception for second home owners unless the other Schengen countries do too, and then it would need to be for all third country nationals with second homes, not just Brits. Right now every country has more pressing things to worry about, so I don't think a few thousand disgruntled foreign home owners will be high on their priority list for some time to come. And yes, the TCN rules do affect my future plans too, so I am just as frustrated by it as others, but it is a direct consequence of the UK's decision to the leave the Single Market as well as the EU, so not Spain's (or the EU's) fault.