The last build that 5/5T received was in May 2020, which is 5 months back. Since the normal software maintenance schedule for stable updates is around 1-2 months, this has left 5/5T users complaining about delayed updates. Finally, Gary C. - the OxygenOS Product Lead at OnePlus, just made a forum post informing 5/5T users about the reason behind delayed updates.
Apparently, the reason behind such a delay is because the product team identified a major bug in the communications module, which affected user experience quite a bit. OnePlus had to engage in a lot of back-and-forth dialogue with the respective comm carrier(s). You can read the source forum post for more information, and even participate in a discussion with other forum users.
Overall, we feel this notice came a little too late as it makes people think OnePlus doesn't care about older devices and doesn't really fulfil the promise of 2-3 years of consistent software updates, but we're glad OnePlus is at least being forthcoming about what's going on behind the scenes. Even though 5/5T will be marked as EOL later this year, OnePlus is still putting the next build (that's been delayed) on high priority. Hopefully, there will be an update soon.
Source: OnePlus Forums