And our profiles we can advertise our websites. Where's the line drawn? Can we put our stream info or commercial sites in our signatures? Or, from another angle, why specifically do you want to curb this content, and why are you considering allowing it in a megathread? I can't really give you any of my thoughts without first knowing yours regarding the prerequisits.
On the face of it, a pinned megathread for something like "matchmaking" is less spammy than 20 threads per page of people asking for match making or a certain type of mod or something. However, things can get lost really easily in a single thread, maybe leaving someone behind depending on the content. Megathreads are great for making outlets for things you don't want to outright ban, but do want to discourage, like "shipping." Let's you keep a lid on something without being called a Nazi for not allowing something on a site.
Some things, however, have loose boundaries and you can no more throw them in mega threads than you can ban them outright. Politics for example, is something that, fundamentally, cannot be banned or excluded. Politics is all about person X arguing with person Y regarding policies on how X, Y, an Z should conduct themselves both in public and in private, which includes A through W. In other words, Stardew valley is political among the right X and Y. With the right X and Y, lollipops are political, even. I've seen politics in ice cream. As a result, a "megathread" for politics would inevitably either require all posts on the site to be in the megathread, or they would find ways to "well, this isn't as political" so it would escape the thread, and if "excused" long enough, people would just push the Overton Window of guidelines for what is and isn't political, offensive, etc. Same with "18+" material, because how much clothing is necessary or excuseable before it becomes "not ok outside of the thread"? There are things i've seen in the game (without any mods or multiplayer) which are too racey for me to feel comfortable showing in the forums given the rules. But the takeway is, how do your draw a line in the sand regarding the nuance between certain topics of what should and should not be in a megathread?