[Android Apps and Games] MaR-V-iN:
There are a lot of ways to detect signature spoofing and it would be possible to spoof those as well, but this is not done in the patch. This means that if Google tries to detect microG, they will be able. I don't think they'll be doing this, because there is no technical reason to do so, whereas signature verification as implemented right now has at least a use case on older devices - on devices shipped with Play Services it does not make sense at all.

The signature spoofing is not designed to hide presence of microG. In fact if you use any app that lists the installed apps, like uninstallers etc, they will show microG as microG, not Play services.

The problem with play services is that it consists of two components: the library and the core app. The core app is what is implemented by microG, the library is included in every app using play services. The library will check that the core app is signed using a specific Google certificate before using it. As I don't possess this certificate I can't sign microG this way. This is where signature spoofing jumps in: it ensures that the signature check in the play services library succeeds with microG. Nothing else.

There are certain differences between signature verification on system and user apps. Android assumes system apps to be legit and because of that the verification process for system apps is reduced. This means that you can modify some parts of system apps without breaking the signature even though this should not be the case. Although this is some kind of security issue that would be helpful for microG, the amount of work required to use it is unreasonably high in this use case and it would be necessary that all updates are pushed directly to system partition. For this reason I built the signature spoofing patch.

For those who know some about Android app development: only the AndroidManifest.xml has to stay the same for a signature on a system app to be considered valid, all other portions like code and resources can be modified.

I doubt that Google will do anything that blacklists microG.

First, consider that Google Play Services is not very agile. Google can't risk that certain apps stop working some day, because they want to block out microG. The singature check in the play services library is in the apps itselfs, which means that changing it in the library will not have influence on apps not using the latest version of play services library. It will take up to years for all apps and users to adopt to a new version of the play services library.

Secondly, I did not hear about any statement from Google about microG or related projects. I think they just try to ignore them and hope nobody will be using them. A change in play services that is specifically made to block out microG could result in the Streisand effect, and this is nothing Google is interested in.

Thirdly, considering that microG and the signature spoofing patch can be modified a lot faster than Google Play Services, it is likely that once Google adds some new kind of checks, we can adopt microG and custom ROMs within a few weeks to ensure that everything works again. Google knows all this and it is unlikely that they are interested in playing cat and mouse, when it is most likely they will loose the game.

I am very sure, that in case Google tries to block out microG somehow (accidentally or on purpose) we will find a solution to come around this. There is no specific plan for this as there are numerous things Google could do.

microG is mostly installed by people with third party custom ROMs. Those are a lot more up-to-date than the average Android phone. I do not have detailed statistics, but I consider the number of Android 4.x + microG users rather low. However it is not that I do not care about old Android versions. If you notice a bug to be present on older versions, you are as well invited to report them.

The old and simple signature spoofing patches should work on Android 4.x as well (tested on 4.4), runtime permissions checks or alike are of course impossible on older platforms. You should also be able to use the Xposed module to spoof signature (FakeGapps) on older Android versions.

If you are interested in some more details on signature spoofing, you might be interested in reading this: http://blogs.fsfe.org/larma/2016/microg-signature-spoofing-security/