Creator's Update for Windows 10 will be coming out soon, sadly, the same can't be say for certain Windows 10 Mobile devices as some of them will never see the new update. Microsoft Lumia devices however, will be covered from Lumia 550 and above. Other than that, the Alcatel Idol 4S, Hp Elite X3 and VAIO Phone Biz will be some of the 3rd party that qualify for the update.
Microsoft have been getting complaints what Windows does not offer the "best possible experience" on older phones. Even with the release of Windows 10 Mobile, it suffered some performance issues when not on recent hardware. Dona Sarkar from Microsoft, has made a statement for those running the new update outside the supported hardware range, saying that they will be running the new update "at their own risk".
It's not new when a OS developer has skip certain devices with the new update because of the advancement in their own OS that older devices will not be able to support using their current hardware. However, Microsoft has a reputation for ending support for older devices too fast and promote its users to buy a more update device, which will increase chances of users changing to another platform