Does removing a custom field option affect existing/resolved issues?

[Joe Pitt]

If you remove the field the related data is also deleted and can't be recovered except by restoring a backup, which introduces all kinds of other problems. 

[Nic Brough -Adaptavist-]

Yes, if you delete a custom field, it removes all data for it.

You'll still have text entries in the issue histories from when it was changed, but the values are stripped off all the issues.