Proper etiquette ?

No, it's not. https://community.ricksteves.com/travel-forum/spain/head-covering-needed

The cathedral in Barcelona (not La Sagrada Familia) was turning away women with uncovered shoulders in 2016. They also told me to remove my sun hat.

A good rule of thumb that almost always works everywhere: look around at what others are doing - read the room.

Works when entering a religious site, or trying to cross a busy street.

We were in Barcelona (cathedral and Sagrada Familia) and Manresa (basilica and cave of St Ignacio) in October 2022. Some women were wearing scarves and covering their heads but they were in the minority and as they made the sign of the cross when they entered I assumed they were practising Catholics. The cave of St Ignacio was full of nuns who had just completed their camino.

I was a bit surprised at what some women wore to Sagrada Familia (short shorts and tank tops) but once inside realised that it doesn't feel like a church at all.

I always have a scarf in my purse or backpack and wear either a knee length skirt/dress or pants when I travel. I have only ever used the scarf for warmth. My niece, however, did miss the Sistine Chapel on a school trip when she turned up in ripped denim shorts and a tank top and was refused entry.

Most of the churches in Spain welcome tourists in all dress. The days of women covering their head are long past.

May not have to cover the head but covering shoulders and knees is critical.

Thank you all. Carrying a scarf makes the most sense and of course “reading the room”. All reply’s were appreciated. Better safe….

Spain has lost most of its religious practices in the past few years, with an overwhelming decrease of Church´s influence in our lives. Mass assistance is less than 10% and churches are quite empty, even on Sunday Mass. Civil marriages are now 85% of total, baptism is less than 30%. We love our traditions and many are related to religion, but with figures of non-believers surpassing those of believers, we do not give much importance to how you dress in church anymore. I have not seen a woman with her head covered in any Spanish church in a very long time, maybe in small towns, but not in cities, definitely.

I'd say it depends on where you're referring to, but I concur with @MikelBasqueGuide.

Talking about "my turf" (Catalonia, and more specifically its capital city Barcelona), I'd say that, historically, we Catalans have never been very religious (in recent centuries that is). This is not to say that during certain times, especially coinciding with authoritarian rule (ie the Spanish dictatorships, when the Catholic Church had more influence over civil affairs) there wasn't an appearance of being much more devoted, and this is not to say either that there aren't committed church-goers in our society even these days... but they're a declining minority.

As per answering your concrete question, except for the Cathedral of Barcelona (which is not the Sagrada Familia, that's a basilica!), there are no requirements to wear anything out of the ordinary when visiting a church. Having said that, and even myself -being agnostic- understand this, it's quite obvious that tank tops and very short shorts are not the most appropriate attire to visit a religious temple, no matter the religion. Some visitors, especially in Barcelona's hot summers, don't realize this is not a museum or any other civil premises but a religious one and sometimes they show up with totally inappropriate attires.

In the case of the Cathedral, it has a sign at the entrances indicating no tank tops, sleeveless t-shirts or shorts (the sign doesn't specify how short, but it appears to mean "above the knees"). Is this enforced? as far as I know, it depends on how zealous the guard at the entrance is. It seems some are more 'permissible' than others. In any case, I have never heard of the need (requirement) to cover the head to visit the Cathedral.

Image: https://ibb.co/3pybfnC

Again, this is while in Catalonia... in Spain, some regions have different attitudes towards religion and requirements might vary.

Enjoy!