Hide unused activity bar icons (view container icons)

frontend-contribution.ts

import { injectable } from 'inversify';
import { FrontendApplicationContribution, FrontendApplication } from '@theia/core/lib/browser';
import { MaybePromise } from '@theia/core/lib/common/types';
import { Widget } from '@theia/core/lib/browser/widgets';

@injectable()
export class ExampleFrontendContribution implements FrontendApplicationContribution {
    /**
     * Called after the application shell has been attached in case there is no previous workbench layout state.
     * Should return a promise if it runs asynchronously.
     */
    onDidInitializeLayout(app: FrontendApplication): MaybePromise<void> {
        // Remove unused widgets
        app.shell.widgets.forEach((widget: Widget) => {
            if (['search-in-workspace', 'explorer-view-container', 'scm-view-container', 'scm-view'].includes(widget.id) || widget.id.startsWith('debug')) {
                widget.dispose();
            }
        });
    }
}

I am not sure, I suggest you ask the Theia community: https://community.theia-ide.org/