According to 2020: a year in closing
In 2020 the number of questions posted on Windows Phone was 30, seven of which were asked and closed. The total number of questions closed that year was eight.
The statistics for the site are not great. Fewer than 3,500 questions have been asked in 9.6 years. That's crazy. How has it survived so long?
I didn't know what the Twitter link referred to, so I clicked, and saw the Stack Exchange site has its own Twitter account with just nine followers.
This is even more remarkable considering that Windows has stopped producing their phones since 2017
Microsoft began to prioritize software development and integrations with Android and iOS instead, and ceased active development of Windows 10 Mobile in 2017. (Wikipedia)
Hey, maybe it has a group of hardcore users that hang out in chat? Let's take a peek…
Erm, I guess not.
The site manages to recruit new users though…
It looks like a small ads (classified ads) page from yonder day :)
How badly does a Stack Exchange site have to do before it is laid permanently to rest?
UPDATE (19 December, 2021)
To be clear, I had and have nothing absolutely nothing against the users or the community of Windows Phone. I have never joined the site and I have had no interactions whatsoever with any user or moderator on that site. Despite the fact that the site is almost defunct there are some nostalgic Stack Exchange users who believe this site should remain open because its answers are still useful.
On the other hand, my question was closed as a duplicate and I must infer that the answer is found in this excerpt from an announcement posted in 2015: Graduation, site closure, and a clearer outlook on the health of SE sites
Tangible guidelines for site closure (aka Reasons why you shouldn’t be afraid)
[…]The precedent was set early on that sites had two options; graduate, or get shut down. This created a lot of unnecessary angst for our smaller and mid-sized communities. Lots of folks on excellent sites assumed that since they hadn’t cleared the graduation hurdle yet, we might come in and turn off the lights at anytime. Not exactly great for morale.
Thanks to many devoted users, it’s grown clear that smaller SE sites can do a great job of maintaining themselves and producing high quality Q&A. Not every site is going to be a blockbuster success, but our small sites are serving their own communities well. We’re proud of you, and we want you here.
What does this mean? If there's enough moderation for a public beta site to consistently remain free of spam, for flags to be cleared, and for our Code of Conduct to be upheld, your site will remain open. However, if community leaders drop off, flags sit without being addressed, and we can’t find any suitable volunteers to step forward, the site gets closed.
As of this post, not a single site currently active in our network is at risk of being closed. Closing public beta sites is a rare occurrence; we expect it to stay that way.
Windows Phone graduated in August 2019 and is therefore not a Beta site. In 2021 there are currently nine open questions with a total positive score of 1.