"Public API" doesn't mean you can't change the API, nor does it limit how quickly extensions or new versions can be added to that API. It just means you have to actually inform people of what you're changing and when.
If a client application refuses to implement functionality, that's on them, not the original developer. If I want the new feature, I'll switch.
These days however, new features nowadays are usually things I don't want. Not strictly outright anti-features, but usually completely pointless "Bob needs a bonus[0]" changes that lets a middle manager put something good in their promo packet. The whole reason why people want compatible file formats and third-party clients is specifically so we can dictate to the originator of those formats and protocols how and how fast they can iterate on their products and limit how bad they can deliberately make them to increase profits.
If a client application refuses to implement functionality, that's on them, not the original developer. If I want the new feature, I'll switch.
These days however, new features nowadays are usually things I don't want. Not strictly outright anti-features, but usually completely pointless "Bob needs a bonus[0]" changes that lets a middle manager put something good in their promo packet. The whole reason why people want compatible file formats and third-party clients is specifically so we can dictate to the originator of those formats and protocols how and how fast they can iterate on their products and limit how bad they can deliberately make them to increase profits.
[0] https://youtu.be/ssob-7sGVWs?t=2748