As in software that’s licensed under a open source copyleft license, but costs money to run that’s beyond an optional request for a donation. I can see this happening with libre hardware since it still costs money to manufacture even if the design is free, but with software, wouldn’t someone immediately fork the project to remove the payment requirement?
Threema is free software but requires a paid license to use it with their network. In this case the license is presumably validated on the server end and this check can’t be avoided by modifying the client.