As I understand it, the apps are free and open source but if they are a part of or make exclusive use of a service that is also a pay service (whether or not it can be used for free) then it will get the ANTI-FEATURES label. In this case the “promotes non-free network” thing. ProtonVPN, Mullvad, IVPN, et al, are companies that charge a fee for their services. Thus their apps will get the label.
OpenVPN does not because it does not offer any services. It is just an app that can be used with any and all and has no connection to the services it can work with. Free, paid, self-hosted. It’s a generic and agnostic network tool.
ProtonMail gets the label because it is aimed at a paid mail service. K-9 Mail and FairEmail dont have the label because they are designed to work with any mail service.
Apps that access YouTube, like NewPipe and SkyTube, will get the label because YouTube is also a paid service (Premium option and all that).
DeepL translator can be used freely but they also offer a paid service so (dedicated) front-ends to that will get the ANTI-FEATURES label.
OsmAnd promotes a subscription service; My Expenses advertises paid extra features. Thus they get labeled.
Like anything, pay attention to the details. Folks have a habit of being put off by the ANTI-FEATURES label without paying attention to what they are. It is information and nothing more. Make use of it as one sees fit but the Chicken Little responses that some people have are unwarranted.