In a mail from Amazon i’ve read they are plannig to shut down PA-API 4.0 by October 31, 2019. So any plugin based on PA-API 4.0 will not work in any way after this date.
PA-API 5.0 is different to PA-API 4.0, it allows only HTTPS and only JSON responses with a different structure than old PA-API 4.0 XML responses. Just take a look on their documentation https://webservices.amazon.com/paapi5/documentation/
Migrating existing PA-API 4.0 to PA-API 5.0 is not a simple job, it costs a lot of developer resources.
