I’m talking about free software, as defined by the fsf. A software is free if it comes with its source code and a license that gives some rights about this source code. Doesn’t matter what horrible features your software has, it is free as long as one can edit them out and distribute their edited version. I am not aware of a definition of free software that includes requirements on the software’s features (like collecting email addresses), maybe you can link to one?

Note that this community’s description is about free culture in general, which is broader than free software. You probably meant that rather than FOSS. Sorry if it seems like I am playing with word, it’s just that I first commented as a response to your question without paying attention to the community.

Now I am not sure about how much it changes my previous answer. If you run an online service, it is probably legitimate to ask for a way to contact your users, as long as you only use it for what they gave you their consent for. If the underlying software is free, people who don’t want to give you that information can still run their own instance of your service. Creating a free software doesn’t come with the duty of maintaining servers that run it, so you are free to impose whatever condition on the ones you run.