Well that won't work because It's not in Hangout section, but I can't know this without searching.
I used this Steam Deck thread as an example, literally just looked at the main forum, picked a random thread and tried searching for it and it didn't show up in any searches. However, this was my consistent experience with searching for threads for a long time now. Maybe it's just my problem, but I don't see how it's a good user experience when I search by relevance and the biggest main OT for a topic is not even in the first results page.
I feel like "relevance" is such a nebulous term that it's hard to really pinpoint what it means in this context. I have no idea how the back-end for this search works and what the "relevance" option is even looking for. Usually it's some algorithm balancing date, frequency of posts, or usage of terms. I'm honestly not sure if it would recognize an old thread like an OT as being more relevant than recent news threads, so I can't comment on that. All I know is that I never use it in any website that has it, from Reddit all the way to here. It's always a mixed bag. In this case the OT was on the second page, so it's not like it's buried, but it's never going to be a human curated list, you know?
I use searches a lot and I look for threads a lot. There's a lot of threads on this forum so I know it can be a lot to sort through some times. In the case of the Steam Deck OT, I'm not sure why it's in Gaming and not in Hangouts because I don't fully understand the OT culture here tbh, but it's actually irrelevant to the situation since your problem would have occurred no matter where it is. Right now if you extend the search to the whole forum, it IS the option that will come up when you search "Steam Deck OT" and I think that's because they actually edited the title from "OT2" to "OT 2" so what I said in my previous post has been addressed. But you can test this yourself. Do a title search on the whole forum for "OT" and then "OT2" or "OT10". Because there's no space, it registers those as different words. This is the correct way the search feature should work, of course. You can't have it able to tell that "OT10" should also come up for "OT" without it also including ANY word with the letters o and t next to each other, like every thread with the word "other" would come up.
So the problem here isn't with the search feature, it's with how people label their threads. That's what your actual issue with finding the Steam Deck OT was and it's why someone trying to find the current, say, Genshin Impact OT, couldn't just search "Genshin Impact OT" and see it as the first result, they'll only see the original one. If you want this problem to be addressed, people just need to put a space between OT and the number for all OTs moving forward. I feel like a lot of examples of people not being able to find something with the search feature usually boils down to something like this, you have to be particular with your wording and most people don't title their threads with the search function in mind.
Not trying to dismiss you or anything mind you, I don't work on the forum or anything so I've got no skin in the game, just legitimately trying to be helpful.