Hello and thanks for your question.  Good news!  We have mechanisms in place on Strava that prevent a bulk upload of historical activities from flooding the Feeds of your Followers.  Your Followers will only see a very limited number of any recent activities.

That sounds like a great feature.

Do you know if it will be working with big amount of activities, like, over 1k? 🙂

Yes, that should be fine.

Sounds amazing! 🙂

Do you know ETA for the functionality?

Will it be available for non-subscribers?

Is there any website to track the feature?

For now I created app that imports TCX files and sync activities with Endomondo JSON files. It's here:

https://github.com/dariusz-wozniak/strava-from-endomondo-importer

When you have the activity ID you can mute it with a second call and set the hide_from_home to true. See https://developers.strava.com/docs/reference/#api-Activities-updateActivityById