Samsung adopted heat pipes with the Galaxy S7 and has used them for cooling on S and Note flagships since. This will continue with next years models according to sources from Samsungs supply chain, which have already started small volume production.
Heat pipes add to the price so they are typically used only on flagships. Which works out fine since its the flagships that pack the most powerful chipsets and need cooling the most.
Theres an alternative vapor chambers. They are some of the best ways to transfer heat and are used in high-end GPUs (think 250W). However, they cost even more than heat pipes and there is perhaps an even bigger flaw.
Vapor chambers are bulkier than heat pipes, which goes against the current trend of thin phones. That said, some makers can now make chambers as thin as 0.4mm. Many manufacturers are reportedly experimenting with vapor chambers but no one has built a phone with them yet.
Schematic of a vapor chamber
They are gaining in popularity on laptops, however, where manufacturers want to offer gaming performance on thin laptops. Even some non-gaming ultrabooks have started using them (e.g. the 10.9mm thick Asus ZenBook Flip S).
Inside sources believe theres a very high chance that vapor chambers will make their way to high-end smartphones in 2019.