Amazing, great job! When and how should apps add themselves to users' app recommendation lists? It's in their best interest, we should have some best practices for that.
Nosta now shows app recommendations on profiles (NIP-89). They look like this:

Click one and you see details, like this:

This new feature should be great for discovering new clients and more easily connecting with others in the places they are active in.
You can create recommendations via nostr:npub1xdtducdnjerex88gkg2qk2atsdlqsyxqaag4h05jmcpyspqt30wscmntxy's fantastic App Manager here: https://nostrapp.link/
Check his profile to see which apps he recommends: https://nosta.me/nprofile1qqsrx4k7vxeev3unrn5ty9qt9w4cxlsgzrqw752mh6fduqjgqs9chhgpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhs8e2cla
This feature will be integrated in a few smaller ways into Nosta in the future. For example, when recommending apps during profile setup. Or linking out directly to the apps a user recommends from their profile.
Discussion
Yes, this should not become a self-promotion tool, but a way for users to express their preferences and let others know where they can be found for different activities. Worst case scenario is that apps show a pop-up right when you log in and try to trick you into recommending them. Your app manager is a great place to offer these best practices to developers. For example, when a developer creates their first app, they could be presented with options on integrating recommendation features in just the right places.
Yes, great idea of adding the guidelines after someone publishes an app.
I think you are right, as you mentioned in DMs, that a good starting point would be for apps to track if some user has visited certain event kind within the app (probably several times) then it makes sense to present them with something like "Enjoying the APP experience with EVENT KIND? Recommend the APP to your followers!" - with a button to add the app to recommendations list for that specific kind.
Global "used for all kinds" recommendations should probably be discouraged.
Does this sound about right?