error TS1243: ‘async’ modifier cannot be used with ‘abstract’ modifier

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Yes you should remove async.

You should not force to use async to the class that implements it. There are other ways to return a Promise, not just async.

Edit:

Since it is not clear for some people why the async is not important. Here a couple of ways to return a promise:

async function iAmAsync(): Promise<boolean>{
    return false;
}

function iAmNotAsync(): Promise<boolean>{
 return new Promise(resolve => resolve(false));
}

function iAmAlsoNotAsync(): Promise<boolean>{
 return new Observable().pipe(first()).toPromise();
}

iAmAsync().then();
iAmNotAsync().then();

Playground Link

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