GitHub Issues do not allow threaded conversations. They are designed to discuss one topic only and don’t handle branching conversations very well.
GitHub Issues can be templated, so new issues are created with text already pre-added, in fact you can go further and make a new Issue look like a form with more advanced config..
GitHub Discussions is another feature of GitHub, they are like Issue but they allow threaded conversations, They are designed for more open ended discussions that can go in different directions.
Email is another format that also allows branching of conversations.
Another difference is that Email is PRIVATE and Discussions & Issues are PUBLIC.
There is a weekly Digest emails of the latest/popular GitHub Issues/Discussions/PRs send to each projects/working groups email. So even if people are not active on GitHub communications are surfaced to them.
If you want to brainstorm ideas or ask open ended questions use Emails or Discussions.
Brainstorm ways to market a product release.
Ask advice for something.
Ask for feedback on something.
Discuss an idea that you are not sure how to break down into tasks.
If you need to keep the conversation private then use Email.
If you need to have a more structured conversation then Issues is the right tool.
With issues you can create templates for different types of issues which means people.
Issues can be open/closed.
Issues can be linked and groups together into collections (epics)
Issues can be added to project boards and managed Kanban style etc..
There is a space for both types of communication in project management, use the right tool for the right job.