I have an 8.5 plugin that was using the IEmailTemplateExtension interface. Not sure where to find this in v10?
Does this mean you have xml email templates? Email template extensions were replaced by the new Token/Template system, and xml templates converted to new template format. Most likely any endpoints from an IEmailTemplateExtension implementation could be converted directly to tokens for use in the new editor for email templates. Here are a few dev articles on tokens and templates: