Fanfiction etiquette is incredibly simple, actually. Don't like it, don't read it. No one is forcing you to, which is why it looks ridiculous to complain about it, especially if you were content warned. AO3 tags + "exclude" is one of the greatest reading tools ever. Use them properly and voila!

The only downside to Cassandra x mad woman is now I have to think for myself in the piano section for swift alert

I'll probably never reach such heights again

But who will have the honour of being the first person I block on blue sky I wonder.