When I first launched https://libretranslate.com I left it open and free to use without authentication of any sort.
This lasted for a while, but subsequently the server came to a grinding halt, due to overuse (too many requests). Based on the logs, a handful of programs were constantly flooding the service for translations (from the same IP, which was easily dealt with using IP based quotas), but in some other cases the flooding were all from different IPs, suggesting that the API had been used in some application that end-users initiate the requests from. Thus the introduction of API keys.
I would love to get to a point where we can provide an open access (no API keys) service, but I’m not sure how this could be sustained financially (setting up servers and load balancers has both maintenance and running costs).
OpenStreetMap as an example serves a large amount of map tiles and provides geocoding services to tons of users without any authentication / charges. But they have lots of corporate sponsors and a substantial budget.