As a tourist, it's a cheat code to be hyper formal. Gets you instant goodwill.

Spanish has actually three levels of formality when addressing someone by name:

<First name>: Informal, casual conversation.

Don <First name>: More formal, conveys respect while still indicating some closeness.

Señor <Last name>: Most formal, normally used in writing.

Using only someone's last name is just plain weird. If you want to speak formally to someone just address them as "usted", it will get you the same goodwill without sounding off.

Don it's very outdated unless you address a very important person.

Don is especially susceptible to huge regional differences. It's as GP described in Honduras and I think as you described in Spain.

I agree it's more likely to be used by older generations but it's not uncommon to hear it around either, especially when addressing or refering to elders

Well, corporation addressing clients would use señor/señora but between older/middle aged men. For anyone under 40 it looks a bit old-fashioned. They would just switch to an 'usted/ustedes' person (formal you/yall, for HN speakers) and no one will feel like they were in 1960.

And even with that, people under 35 will just stay at the informal (normal you) "tu" case.

Same.

The only situation where I call people by their last name in my language is when it's their nickname. Like there were two "Johns" so we call the second one "Smith".

In Portugal it's very associated with military service. I don't think I've ever been addressed by my last name here.

In Spain maybe if you are something really important or being addressed on very formal environments. Maybe a boss/manager from a corporation addressing clients...