An incomplete list of places that traveling trainers should avoid:
Burned Tower of Johto - Severity: 6/10
Long ago, there were two towers in Ecruteak City. One of them burned, though, leaving behind the Burned Tower. While the main floor is pretty stable, and people often come here to leave their respects, do not go to any upper floor. The wood has been heavily damaged, and there have been many reports of it giving way, causing people to plunge several stories down.
Lumiose Catacombs of Kalos - Severity: 7/10
Lumiose has many, many kilometers worth of catacombs beneath it, and no one's come even close to fully exploring them. Even with advanced navigational equipment, people have gotten lost in these tunnels, and some never made it out. The ghosts within the catacombs might point you towards an exit, or they might send you even deeper. Unless you're extremely prepared, it's best not to delve into these catacombs.
Sea Mauville of Hoenn - Severity: 8/10
Aside from, if the legends are true, being one of the most unethical and hostile workplaces while active, Sea Mauville hasn't gotten any safer since being abandoned. There's twisted metal everywhere, some places have completely flooded, and high-pressure pipes can burst with no warning. I know it's a popular place for danger-seekers, but it really should be left for nature to reclaim.
Galar Mine No. 3 of Galar - Severity: 9/10
You might be thinking, "Doesn't Galar only have two major mines?" And you'd be correct, in the present day. But years ago, Galar used to have a third major mine. The reason it's been abandoned is because of toxic fumes that have filled the entire mine, it having a penchant for sudden cave-ins, and, reportedly, a never-ending inferno at the very bottom of the mine. You'd be safer going spelunking almost anywhere else.
Area Zero of Paldea - Severity: 10/10
The fabled Great Crater, the one place forbidden to students of Naranja-Uva. There's good reason for that. Area Zero is absolutely filled with extremely hostile Pokemon that won't hesitate to maul you, giant falls onto sharp rocks, and an ever-present mist that can turn carbon and silicon into crystal. Stories speak of treasure somewhere within Area Zero, but whatever it is, if it even exists, it's not worth it.