By far the most explicit has been the Bank of Canada which, given the uncertainty during the crisis, gave a definite timeframe until which it would keep overnight rate at the “current” level, subject of course to the outlook for inflation. The ECB has traditionally been less explicit on the ground that policy commitment is not possible in an uncertain macro environment.
Second, we do not need to risk unsettling financial markets via a passive runoff or active sales of bonds we hold on our balance sheet, given that we could proceed with the necessary withdrawal of accommodation in other ways.