It's hard for me to tell now if promo threads are less effective generally or if promo threads are less effective for me through a combination of "pretty much everyone who's on the forum who is interested in reading my comic is reading it at this point" and "When you're over a few hundred subs, picking up a sub or two is much less noticeable than it was when you had under a hundred".
I think they're less effective than they used to be through a combination of the forums being harder to find, meaning fewer new users coming through, and the ability to hide categories meaning that a lot of people now completely mute all promo threads.
Overall I always found that the ones themed around a specific genre or subgenre like "share GL stories!" or "Share isekai novels!" worked the best, and where you post sample content, like "post your latest page!" "share your cover!" etc. are also pretty good, while anything where it's unsorted and everyone just posts a link and runs are the least effective kind because there's so little content to catch the eye and everyone there is putting in the absolute minimal effort entirely focused around promoting themselves.
The thing about using promo threads effectively is, it's all about quality over quantity. If everyone ignores you in a few promo threads, chances are, posting the same thing in other promo threads won't solve the problem. For example, early on, I found when I posted my link, people weren't clicking on it. I fixed the problem with one simple change. At the time, I thought the "banner" for a series just decorated the page, but on discovering that it also is used as a thumbnail for forum links, I changed it...
Immediately all my posts in promo threads were getting way more clicks and it resulted in more subs. So a factor people often overlook is "how nice and well-presented or appealing looking is whatever you post in the promo threads?" and it's a much more impactful element than "how many promo threads did you post in?"