OnePlus is currently in beta and to stay within the two months update window they promised, that global update should come in the next week or so. TMobile is about 1 month behind OnePlus.
also and this is just my opinion, but given the fact that RCS is an umbrella term when in fact it's really Advanced Messaging with file limitations at 10 megabytes, and the fact that that is not working correctly on the T-Mobile variant currently, there may be an extended wait while TMobile tries to fix the last update.
So unless T-Mobile has an unforeseen trick up its sleeve you can count on staying on the January security patch until May or June.
It's rather unfortunate that the OnePlus 6t only contributed to 2.4% of overall quarter for 2018 sales for Tmo. That of course lends itself to theories that it's not the most focused upon handset for further updates from Tmo.
Android Authority: Report: OnePlus 6T made up only 2.4% of T-Mobile phone sales in Q4 2018.
I would expect that most of the features in beta will come to stable and then to the T-Mobile variant. The T-Mobile variant of course did not get deep Duo integration but the global version did not get Advanced Messaging.
But I would expect things like the updated weather app with 12 and 24 hour format as well as screen optimization and battery improvements to be part of both.