Examples:
Hackenproof ToS: Terms and Conditions 2025 | HackenProof
“Hacken OÜ reserves the right to change all Content and features of our Website, at any time without notice. While our Website is currently free to use, we may begin charging for use of all or part of its features at any time, without notice to you.”
Alternative title:
The service may stop being free at any time without notifying the user.
I think this is more relevant than ever now, with Reddit charging for API access and Unity charging developers for every time a user installs one of their games.
Everyone understands that the world changes and previously free or cheap offerings might see price increases, especially if they were funded by donations.
But there should be at least be a reasonable lead time (f.e. six months for existing users) and an attempt to work together with people who built their infrastructure on your service.