First, do nothing.
OpenQA.Selenium.Support.PageObjects library is actually deprecated you don’t need to do anything. It could be years before the library is actually deprecated, and longer before it ceases to be usable. As long as it works for you, there’s no downside to continuing to use it.
Then learn how the library works. If you can’t find a viable alternative that has the functionality you need, you’ll need to code your own – which means you will need to understand how the library works and what you need to code your own.
Alternatives – If you can find an alternative that has most of what you need, you can learn to extend or create your own helper classes to finish off the functionality you need.
If you’re really ambitious and you can take your own fork of the library source you can maintain it yourself. Whether you think it’s worth the time investment or not depends on how extensively you use it and how good a programmer you are.
Above all don’t panic. The library won’t cease to be useful for some time after it’s gone out of maintenance. Usually the most you need to do with old libraries is modify your app.config files to force them to use more up-to-date versions of any references they require. The only time this fails is when the new versions of the references have deprecated and dropped functionality that your library requires.