Letterboxd should add every season of True Detective on their site. Each season is their own condensed story, because it's an anthology series.

Letterboxd has anthology series' on their site. Such as Love, Death & Robots, Dekalog, Black Mirror & even Sherlock.

They explained since it's an ongoing series that they can't add it on the site. That doesn't make any sense because two of the shows listed are still ongoing. 

Each season of True Detective are basically mini-series' which is what Letterboxd has. So I think it would make perfect sense for them to add the show on their site.