According to a post in the message center not long ago, the shared channels feature in Teams is about to roll out in GA after being available in preview for some time.
Although shared channel seems to be available by default in the Teams client, please note that this is only an internal availability, unless the admin of the tenant has enabled/allowed the b2b-directconnect in Azure AD.
I recommend reading up on this article in order to familiarize yourself with the configuration. It is not enabled by default, and there are several options available. A good approach could be by opening to specific partners and not allowing "everyone". If the company has invested in AAD P1 or P2 licenses, it is also possible to trust the partner's MFA and Compliance checks in conditional access, easing the usage for users.
This feature adds to the mix of external user access. It does not replace the guest user feature, nor anonymous access to teams meetings. But the real benefit of shared channel in my view, is how it adds the channel in the external user's existing list of teams and channels and does not require a tenant switch.
According to the message center post, this should be rolling out mid July and the following weeks.