The checked attribute for setting the default radio button has been there all the time, from my recollection at least since Material 9.x (and possibly before).

Can you try the following:

In you TS code:

Set your favorite season selection to a constant string value:

favoriteSeason: string = 'Spring';

Then in your HTML set the checked attribute in the ngFor loop for each mat-radio-button control as follows:

[checked]="favoriteSeason == season"

So, your radio group will look as shown:

<mat-radio-group
      aria-labelledby="example-radio-group-label"
      class="example-radio-group"
      [(ngModel)]="favoriteSeason">
    <mat-radio-button
        class="example-radio-button"
        *ngFor="let season of seasons"
        [value]="season"
        [checked]="favoriteSeason == season">
        {{ season }}
    </mat-radio-button>
</mat-radio-group>

The above should give you the desired default selection.

Edit: You might have experimented with the above and found out that the checked condition isn't needed if we set the initial value in our TS code.

Try the following:

In your TS code, don't assign the binding variable. Leave it like this:

favoriteSeason: string;

Next, add the following HTML button:

<button (click)="setValue()">Press Here</button>

Now, define the button click event setValue as shown:

setValue() {
    this.favoriteSeason = 'Autumn';
}

When the above runs, there is no initial selection. When you click on the button the season will be set to 'Autumn'!

The above shows that you can set the default radio selection using either code or with the checked attribute.