From the link you provided, library is still there and you are free to use it. With disclaimer that it is no longer maintained, and possible there could be other such libraries with more community support, which would be better, more future-proof candidates to base your design on.
Originally, it seems that this code was intermixed with other .NET code for Selenium, and developers extracted it into a separate entity, so they separate the core part which they do plan to support, and PageFactory, which they don’t intend to support in the future.
Or, in near future someone from existing users might decide that their previous investment in it is too valuable and decide to maintain it.
It is ALWAYS this risk you have to evaluate when picking a technology/library in any of your projects, and you have to ask: How healthy is this technology? Does it have support? Will it’s support continue in timeframe my project will use it? You don’t want to be last person using some obsolete technology.
At minimum, try to find if there is a list/forum/whatever for the current users and/or developers for this deprecated library. Maybe there is somewhere, and they plan to announce the support soon? Google for blogs post from developers/committers, etc.
One problem with using such generic name for a project (“.NET PageFactory”) is that it is hard/impossible to find relevant hits in google search results.