I wonder, can't I create the main Slack profile as a developer without creating a team which can later be shared to any team I'll be invited to? I want this because each time a new team sends me an invite, I have to register all over again, although it's still the same Slack website.
Slack – My profile on Slack for different teams — am I able to create it only once and use for all teams
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I also use /s after:yesterday which lists all posts today, in reverse chrono order.
The 
symbol next to the channel name on the left panel indicates that the channel is a "multi-workspace channel":
Go to the channel's details panel on the right to see which workspaces have access to the channel:
Example where the entire organization (i.e., all workspaces) has access to the channel:
Best Answer
No. You Slack user and profile is always part of the individual Slack team / workspace. Slack does not support global users.
As I understand that is part of Slack's security architecture, where it's not possible to see other Slack teams unless you have been explicitly invited.