You can absolutely get brain damage by the use of inhalants, and while sometimes it's reversible, sometimes it's not. When I was around 13 or 14 I started abusing chemicals in this way. I did it almost every day, sometimes more than once.

I had a scary experience when I had inhaled a little too much and couldn't breathe. I ran out of the bathroom and found my sister, who's first response was "Are you high, because your lips are blue". I turned out to be alright, but it terrified me. Following that my memory didn't work at all; important details (my family's ages and names, my own age, what day of the week it was, etc. I had a headache and I was constantly nauseous, unable to eat.

That was a long time ago, and I'm completely fine now. But everyone is different. Long term brain damage is definitely a thing with inhalants.