The following code renders with no event handlers attached. I've verified that this.evchange and/or this.evinput are true. The events work if I set them up more verbosely. But from reading the Conditionals section in the docs, this should work.
let evc = (this.evchange) ? html`@change=${this._changed}` : '';
let evi = (this.evinput) ? html`@input=${this._changed}` : '';
return html`${this.flabel} <input .value=${this.fvalue} ${evc} ${evi}>`;