I don't particularly think that its old enough to be retro - its kinda a weird situation with a platform with no real support and no real potential for growth. With a lot of these "retro" systems there's a vibrant ecosystem, with real new build or new old stock, refurbished or emulated systems.
Looking at the timeline for windows mobile, the tag refers to the OS that was introduced in 2000, which is 20 years and the last release was about a decade ago according to wikipedia.
Windows phone is contemporary with windows 7 - which is still a 'modern' windows OS, and even XP isn't particularly retro, though part of that is that XP was in commonish use when Super User was old. Windows 10 mobile was also in active development until 3 years ago and EOLed this year. You can even run UWP software on it in theory.
I'm not sure there's a "future" for the platform, less so than even classic platforms like commodore or amiga and as a "modern" closely integrated system, its unlikely that these machines will have the same level of repairability most proper retro systems have.
Without MS's support, as a closed platform, on the short term, I don't particularly think there's a potential for a community outside niche holdouts for windows mobile.
I don't think a merger needs to be for the 'sake' of merger - and that if there's a remaining Windows Phone community - retaining their own site seems to be an advantage in terms of discoverability. The lack of activity is because few are enthusiastic about it and that seems a natural decline considering the short life of the platform.
I'd propose an alternate approach. While I'd love to say "maybe have a generic site for gadgets" - there's several "niche" phone platforms such as kaios, sailfish or even mainline linux that are not served at all under the Stack Exchange umbrella. I feel like rescoping Windows Phone to be a more "generic" Smartphones.SE would be an option that keeps the current community in place, hopefully brings in fresh blood, and retains a space for folks with expertise in less common smartphone platforms.