for example:

old system had separate auth backend for:

1. jira cloud & confluence cloud
2. bitbucket
3. trello

new system:

1. atlassian cloud (e.g., jira, confluence, trello, bitbucket)

i'm not sure if this migration is finished enough to add yet

It’s appropriate to add one of these rules if different websites share the same backend credential system. If that’s the case, then we should add a grouping. If that isn’t the case yet, we should not.

I apologize for being obtuse --

Here is a clearer example.

https://id.atlassian.com/login
You can sign in with an Atlassian Id.

https://trello.com/signup
You can sign in with an Atlassian Id or a legacy Trello account. New accounts are Atlassian Ids.

I haven't been able to test this right now. However, if you login with a trello account it should prompt you to create an Atlassian Id.

From https://help.trello.com/article/1197-using-trello-with-an-atlassian-account#9 (updated updated on April 27, 2020)

Can I opt-out of migrating my Trello account to an Atlassian account? Is there a way to undo the migration once it’s been completed?
Over the course of the next year, all Trello accounts will get an Atlassian account. We do not offer an opt-out of this process, nor is there an option to reverse the migration once it’s completed.

Upon further reflection, I think a group of "id.atlassian.com" and "trello.com" would be a great idea in a few months.

You’re fine! Thanks for clarifying this. Feel free to bump (comment in) this issue once that change has happened and this seems worth doing!