Coming up on month 3 now! I haven't been able to get a reply from Support, but I'll just use this thread to update you guys if/when a Related tab appears and the statistics as to how long it took/how many downloads/reviews etc until it appears, so at least it can helpful to folks in the future!
Currently: 2.5 months since game launch, 1.3k downloads, 310 collections, 31 reviews, no tab.
Investigation: Projects from other accounts that are up for 3-4 months with 5+ reviews have Related tabs; some projects with less than 5 reviews also have tabs.
Hypothesis: Related tab appears after 3-4 months, so it's time-based. However, I've seen games that've been around 5+ months with no Related tab, these don't have reviews attached to them, though. If anyone has a game that has a related tab that was published less than 3 months ago, please let me know and I'll add it to my findings.
Thanks guys, hope this ends up being helpful to someone in the future!
EDIT: It happened!! Total stats: 85 days on itchio since launch, 1.4k downloads, 334 collections, 32 reviews! Tab acquired! :)