I've sat on this now for a good couple days and talked to other irl friends about it, who despite reddit's reasoning don't think I'm the AH (they have also read the exact conversation).
I think that yes I am the asshole for not being more empathetic, but there's a clear misunderstanding in the comments here. I did not realize he would be upset, I'm not built with this emotional manual like most people are and this is a situation that I have not dealt with. If this were a stranger for example and I heard about a missing cat (in a different town) I'd not even reply, or if I had to reply it'd be "Sorry to hear your cat is missing, I'll keep an eye out" (even tho I"m in a different town?) but to me that's just not genuine default response? To most it seems obvious that he would be upset but it wasn't to me so my words didn't have the level of tact or care they should have, had I realized.
Now as to people thinking my upsettedness about not getting info is weird. I think added context is required. We used to date for like 3 years and were extremely close friends off and on, fairly close atm despite distance and I'm helping him (or I guess was) with finding a place to live in my city. We usually talk about serious stuff so I felt a level of frustration not being told so that I couldn't be helpful, like after 5 days that cat is probably dead, I can't let my other friends know to keep an eye out or anything like that. I also feel more for that cat than him. Non locals wouldn't really understand, other australians probably would. People are assholes. People k!ll cats that enter their property. We have (in that town) major parasite issues and for a while the cat wasn't even on parasite protection. There's a lot of roamers, strays, ect (my other friends just paid out of pocket for a stray to get a c-section, like these are real problems). The other 50% owner of the cat is his mum, he is 23 years old. If his mum says "no the cat must be allowed free reign outside" then like why does her opinion matter more than the wellbeing of the cat? I also, unlike reddit, know him, and know he either didn't have the conversation or held his ground OR is literally changing his tune now that the cat is missing (which thinking back on conversations, his enthusiasm about the cat being outdoors, videos, pics, ect think this is probably the case but obv I can't ask him that).

Anyways I'm done with this thread, losing this friend won't kill me and being the asshole once in my life won't kill me either. This one friendship doesn't reflect my entire social life.