Ever since Jamie fixed bug 1585907, screen reader users are more aware when reader mode is offered. However, reader mode seems to be offered in contexts where it does not make much sense, like when in an application like Slack. While some parts of it might warrant reader mode, since reader mode applies to the whole page/application UI, it does not make sense to offer it in these circumstances.
Updating the Accessibility Team's impact assessment to conform with the new triage guidelines. See https://wiki.mozilla.org/Accessibility/Triage for descriptions of these whiteboard flags.