OnePlus has no plans for implementing expandable storage in its devices according to a response from Carl Pei on Reddit. Recently Carl Pel did a Reddit Ask Me Anything, and one of the questions that came up was whether or not OnePlus would ever integrate expandable storage with its future phones, and Pei says he feels like users never give this kind of feedback. Whether or not a majority of users feel that expandable storage should be an option, Pei sticks to notion that internal storage use only is a better way to go due to some improvements over expandable storage.
One of these reasons is that, according to Pei, internal storage is faster than expandable storage which should mean that users will have an overall smoother experience when accessing any of their files on the device. This is definitely a benefit if everything is working correctly because no one wants to sit around waiting for their storage to load their files. OnePlus already offers a huge amount of internal storage on its phones which is a reason why Pei feels like customers never really ask about or insist that OnePlus offer expandable storage as well.
This makes sense if you think about it, as OnePlus most recent flagship, the OnePlus 5, as well as the OnePlus 3T, OnePlus 3, and OnePlus 2 have all offered a 64GB option for internal storage, as did the original OnePlus One eventually. On top of this, last year's OnePlus 3T and this year's OnePlus 5 also both offer a 128GB model, so really there is plenty of storage space for most users, though there are likely to be some who might need more. At least one user seems to agree with Pei that just internal storage use is fine, while others seem to feel that having a microSD card slot would still be beneficial for transferring files between devices. The nice thing is that it's entirely possible to execute a similar type of task using a USB Type-C flash drive, or USB Type-C to microSD adapters. Pei doesn't state that OnePlus will never implement expandable storage, but he does mention that at least for now, it's not in the cards for OnePlus phones. That does suggest the company is at least considering it as an option though, or that it has thought about it.