Hi Elizabeth, I see that you already belong to three projects, so you could follow the links from those to read up on a few of them that cross your path. It is always a good idea anyway to familiarize yourself with the project universe that exists in your area of interest.
You can see the global Projects top-level category for a fairly comprehensive list of all of them, or at least to the main gateways for various ones that are not yet categorized there.
The policy change is that you "must find out if there's a related project".
That is not a very high bar to cross. And if you do nothing for that on your own, you will be approached by Volunterr Coordinators, to direct you, after you certify pre-1700. The idea is to add awareness about which project territory you may be working in, for your benefit and for the benefit of the project.
Theoretically, every pre-1700 profile does, should, or will, be templated into a project. Otherwise, it is likely garbage that should be deleted, or blocked before creation. But any valid person who is documented as living before 1700 has millions of direct or close relation descendants. So over time, any such profile needs to be proven valid, get improved, protected, and brought under the umbrella of a project.
If you are creating good profiles, then project members will be taking these actions on your good profiles. So this is just a sensible way for all interested parties to be in contact with each other about it.
But if somebody is creating or is oddly editing profiles that are covered by a project's territory, then those project members need to be able to quickly know, so that they can mitigate any tree damage, and to make sure that the profiles comply with the project standards.
The projects all welcome good profiles. But once a good profile exists, it is important that it stay a good profile, and not get swamped in waves of new duplicates that are maybe not so good. So as I see it, this is merely a helpful step in the tree integrity process.