Note v8a is the 64-bit build, and should be considered the default choice.
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The new default is the RedReader client ID.
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New Enable Overrides toggle, off by default, to still allow overriding the client ID, user agent, and redirect URI
Most users' client IDs have been removed by Reddit. If yours was removed, log out of each account and login again.
Check your https://old.reddit.com/prefs/apps/, some still exist. The override will still let you use it.