What are the plans to support the Raspberry Pi 3B+?? Are there plans to release a version for the hardware?
I see this question quite a lot on this forum. The reason Ubuntu MATE doesn’t boot on the Pi 3B+ is because it uses old bootloader files (bootcode.bin, *.elf, *.dat) incompatible with the SoC used in the 3B+. The 3B and 2B v1.2 have the first revision of the BCM2837 SoC, known as the BCM2837A0. The 3B+ and 3A+, on the other hand, use the second revision of the SoC, BCM2837B0.
Whether Ubuntu MATE may be officially updated at some point to support the BCM2837B0 I cannot confirm. There hasn’t been any official news for weeks about how development on the 18.04-based release is going.
I don’t own a 3B+ and therefore cannot provide any reliable advice for adapting the current 16.04.2 image to boot on the 3B+, but there is a tutorial on this forum about how to adapt the 16.04.2 image to boot on the 3B+ as well as upgrade it to 18.04. These steps require some manual intervention though.