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

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