I often wonder what the business model of open source Bitcoin wallets will be. They have a number of options before them (#944967), but this maintenance fee is something I hadn't heard of before.
Here it looks like the Wix Toolset Project is trying to introduce the concept:
To ensure the long-term sustainability of this project, we are introducing an Open Source Maintenance Fee. This fee is required to be paid by all consumers of the WiX Toolset project who generate revenue. You can pay via GitHub Sponsors.
Sadly, I don't think it will catch on in Bitcoin land.