I don’t know how widespread it is outside of metal, but I’ve been seeing more and more bands offering tapes. Sometimes a release is only on tape, other times the tape might be $6, the CD $15 and the LP $25, so there are different ties available for those who want a physical copy. I probably got 10 tapes or so within the last year.
Yeah, it’s just tradition at this point, though I feel like native speakers really try to oversell its usefulness when someone questions if the opening signs are necessary. People act like they routinely need to read text written like the Cartas de relación out loud, and thus, need the additional warning lest they get lost in the long, multi-clause sentences. Like, I could understand if you had to read something like
out loud on a regular basis, but even contemporary literary Spanish doesn’t tend to have nearly the same amount of sentences that just go one for half a page, much less the sort of stuff people would write to each other normally.
As you’ve mentioned, other syntactically similar languages do just fine without them, even including other Romance languages spoken in various regions of Spain. The only exception I’m aware of is Asturianu, which apparently also uses them, though apparently they’re optionally allowed in Galego Real Academia Galega. On page 38 of the PDF, it says they’re entirely optional if you want to facilitate reading by including them.