No Typescript Code Completion

Sidenote: I'm using the TabNine plugin to supplement code completion suggestions.

But I've uninstalled it once and the issue remained.

Please share a complete code snippet (as text/file(s)) the issue can be repeated with

You could try it with the IonButton props:

import React from 'react';
import {
  IonHeader,
  IonContent,
  IonToolbar,
  IonTitle,
  IonPage,
  IonButtons,
  IonMenuButton,
  IonButton,
} from '@ionic/react';

const AllGoals: React.FC = () => {
  return (
    <IonPage>
      <IonHeader>
        <IonToolbar>
          <IonButtons slot="start">
            <IonMenuButton />
          </IonButtons>
          <IonTitle>All Goals</IonTitle>
        </IonToolbar>
      </IonHeader>
      <IonContent>
        <IonButton fill="clear" expand="block" size="small">Button</IonButton>
      </IonContent>
    </IonPage>
  );
};

export default AllGoals;

It's a known issue, please follow https://youtrack.jetbrains.com/issue/WEB-49713 and linked tickets for updates