The support comes at the same time for the simple fact that, basically, it is a single smartphone, called by Xiaomi with two different names (it seems that Redmi 5 Plus is the name for the Chinese market, while Redmi Note 5 to the international). After unlocking the bootloader, a modified recovery is the necessary next step to vote for modding, so good news.
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Although the two smartphones have been launched by Xiaomi with Android Nougat on board - so no native support to Project Treble - the community has already cooked a version of LineageOS 14.1 rather stable, being a work still in its early stages. So if you were looking for a recent smartphone, with good technical specifications, with a price that is more affordable and supported by the modding community, Redmi 5 Plus / Redmi Note 5 could be an alternative to consider.