iOS shows every custom keyboard the same warning when you grant Full Access, and it's written for the worst keyboard you could install, not the one you did. Here is what it means for Clink.
What it unlocks: haptic feedback on keypresses, clipboard history, and custom key sounds. It also lets the keyboard read the storage it shares with the Clink app, holding your themes, layouts and learned words, so the keyboard you set up in the app is the keyboard you type on.
What Clink does regardless of the setting: no keystroke logging, no analytics, no accounts, and no network on the typing path. Prediction and learning run on the phone. There is nothing to send and nowhere to send it.
And if you'd rather not grant it: don't. Clink works without Full Access. You keep every theme, every layout, swipe typing and prediction. Only haptics, clipboard history and custom sounds need it.