We deduced this earlier and now we have an official confirmation that some devices (or rather all the devices except Lumia 830, Lumia 930, Lumia 1520 and Lumia Icon) won’t mostly receive any firmware update with Lumia Denim.
As per the official FAQ reply,
Lumia Denim is the marketing name for latest Lumia software update, which is a combination of firmware, OS (operating system) and Lumia exclusive applications. Exact contents coming with Lumia Denim vary depending on the device model, and some devices are not requiring new firmware for the new features to be enabled, so firmware can in some cases remain unchanged.
Lumia Denim anyways was not supposed to bring anything worthwhile to any other devices than Lumia 830, Lumia 930, Lumia 1520 and Lumia Icon. So, for those on latest version of developers preview, which is 8.10.14219.341, there is hardly anything to look forward if you have any other device than Lumia 830, Lumia 930, Lumia 1520 and Lumia Icon. You mayn’t even get the update, if you are on developer preview and have devices other than Lumia 830, Lumia 930, Lumia 1520 and Lumia Icon.
For those who are on Lumia Cyan, and rocking a Lumia 830, Lumia 930, Lumia 1520 and Lumia Icon, you need to read both Lumia Denim + Windows Phone 8.1 GDR1 changes on our dedicated page. Otherwise if you have Lumia Cyan on any other device, check our dedicated page for only Windows Phone 8.1 GDR1 changes.