Bu using BetterAMP plugin all your posts and URL's in your site will have a AMP version but when you deactivate the AMP plugin that URL's will return 404 error. This plugin will redirect all that AMP URL's to the relevent main URL and will fix the 404 and SEO issue.
This plugin will work for following patterns:
- yoursite.com/post-slug/amp/
- yoursite.com/amp/post-slug
All AMP URL's with 404 error will be redirected to relevent page by using 301 Redirect and your site will not return 404 error.
This plugin support following plugins:
- Better AMP by BetterStudio https://betterstudio.com/wp-plugins/better-amp/
- WP AMP official plguin
- AMP for WP plugin