I very much doubt that /e/ devs will allow themselves to skip one of the most demanded devices of the last year, which OnePlus 6 truly is.
However, if to take OnePlus 6 right now, I don’t think it would be a tragedy to temporarily flash it with unofficial eOS build or LineageOS 16 for microG with Bliss launcher.
On the other hand, if to buy 5T, you will get the more obsolete device that you will not be able to change as easily as it’s firmware.