IRC is currently our default communication tool for most projects, and it is quite appropriate for many of them. But it is not a mobile friendly tool, and not ideal for adhoc but timely mentoring.
Phabricator's Conpherence is the other tool available and used by Wikimedia, and it is slightly better in some ways, but it doesnt even have integrations with other Phabricator components. It is suitable for adhoc discussions, but is not designed for a team 'room'.
It is also terrible on mobile and that wont be fixed any time soon (the devs tend to close mobile related bugs at will if they cant reproduce the bug on their own device or if they dont like the combination of components used).
The Wikimedia Conpherence installation doesnt have notifications enabled (T765: Enable notification server (real-time pop-up notifications) in Phabricator), but progress is being made at T112765: Phabricator needs to expose notification daemon (websocket).
Conpherence gets especially unusable for a long conversation over many months on a mobile phone. For one GSOC, we needed to create a new Conpherence because the old one was unusable on mobile.
For GCI, Google Summer of Code, and Outreachy (and others), we should be offering participants and mentors the best technology in order to maximise the outcomes and reduce the failure rate. In a mobile world, that means a good mobile app is mandatory for mentors to be available for the participant in a timely fashion.
Slack and Gitter are very easy options, but requires trusting a third party. Many other GCI orgs are using these types of tools.
Preferably Zulip, which is also a GCI organisation. It doesnt have as many integrations available, Mattermost doesnt have Phabricator integrations either (https://github.com/mattermost/platform/issues/1973), and Zulip is moving much faster than Mattermost, and has a technology stack that WMF is more accustomed to.