As Paul has said I don’t think there is anyway to click a button and unfollow everyone who is not following you.
But then in theory you don’t need to unfollow ALL of them you just need to unfollow enough to the point where you can start following others again. Again in theory if you have just been stopped from following anyone you should be on exactly 5000 in total (following and non-accepted requests). So unfollow 10-20 and see if you can then follow someone new.
Some suggestions as to how to quickly identify who to remove, which you may already have considered.
Go to your following and start at the very bottom, they are ones you have followed longest. Look at their activity and if it is more than 1 or 2 years old just unfollow them, they have given up.
When you check out a ‘Following’ things to quickly look for:
If they have 1 or more in the ‘In Common’ section good chance you are following each other - leave them and move to next one.
If they have plenty more following them but don’t follow many themselves they probably never followed you back. If they are not following many, ie less than 100-200 (you can check higher numbers if you wish) the easiest way to check if they are following you is to click on their following list and scroll down through their list. DONT look for you own name look for the gap on the right hand side it is much easier to pick up.
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Hopefully you can quickly unfollow enough to allow you to start following others again.
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EDIT: afterthought, scrolling to the bottom of your following list will take you some time. Once you start the process of looking at those you are following and unfollowing them do not leave your following list as it will take you right back to the top when you next go in. Details of those you are unfollowing will not be removed until you exit following list and come back in again. I don’t have many on my following list and I’m not removing any of them so I can’t test exactly what I have suggested. You might want to try removing 3 or 4 nearer the top of your list to confirm everything works without having to scroll to the very bottom first time.
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Looks like this all came about in April 2020 but Zwift never answered the same question you have raised.