“If this fic contains something that squicks you, personally, out, don’t read.”
Still a little vague but better
“If this fic contains something that squicks you, personally, out, don’t read. If this fic contains content that is problematic, especially to a whole group of people, I.E. the -isms, and romanticizes it, don’t ignore it. That fic is actively doing harm.”
“But, Avaquet, stories need problems to be a story! I have to make my story problematic!”
Yes, problems are fine. Conflict is fine. The issue arises when you as the author begin to romanticize problematic themes or statements in your works.
Here is a big difference and two examples:
A: “Character A walked down the streets of City. Various people reacted differently to Character A. Some clutched their purses and looked scared. Others snarled at Character A and snubbed their noses. Character A scoffed right back at them, questioned why they were so judgmental and shook their head.”
Here we see a problem, A is being judged harshly. Is the problem romanticized? No. It’s not.
B: “Character B walked down the street and met up with Character C. Character C had been the bully of Character B for a long time, but C had realized their mistakes and that they actually loved B! They kissed in the alleyway and B forgave C of all their heinous actions!”
This is...problematic. It romanticizes bullying in this case. Which yes people can  discern fiction from reality but, stories stay with us. How many of us refer to our favorite stories? Be it for romantic ideas or why one would be afraid of something because of a horror film? It does effect us. Look at the effect Jaws had on the public, a famous example on how fiction did effect reality. People killed sharks way more often. There became a massive fear. Hell, the rollercoaster scene in Final Destination gave a fear of rollercoasters to a ton of people even though about 90% of that scene was all CGI because they couldn’t even come close to recreating it.
Look, it’s okay to have problems in stories, it’s how they’re driven. It’s okay to not have a good ending and let the “problem” win. What’s bad is when the problem is romanticized or celebrated. Sexism, racism, abuse, bullying, etc. (inaccuracate BDSM and Kink practices I’m looking at you 50 Shades, which is another story that impacted reality) Treat these with respect. Get a sensitivity reader if you must. But if you’re going to let the problem win, show why that’s bad. If you want a redemption arc, understand that simply acknowledging that the character did wrong is only the first step and also understand that at the end of that redemption arc, there’s still probably going to be people that don’t like said character.
I’m all for having fun and doing what you want, but sometimes your safe space is a harmful place for others. Our actions impact those around us and sometimes across the globe without even direct contact at times. We’re not in a vacuum.
Something that you don’t particularly like. Makes you cringe. Makes you go eugh. Squicks are personal usually. My squicks include baby fics, so I don’t read those cause I know I won’t like it.
But y’all, racism, sexism, ableism, etc. those aren’t squicks. That’s problematic.