Make a post sticky after x days

I assume as you are in ‘Developing with WordPress’ you are happy to code.

Fist you will want a cron job, running say daily

see https://developer.www.ads-software.com/plugins/cron/

then in the job you will need to identify the posts that have become eligible for stickiness by comparing the post date. Then you will need to add to the list of sticky posts. The sticky posts are stored in an option ‘sticky_posts’.

see https://developer.www.ads-software.com/themes/functionality/sticky-posts/ to understand how sticky posts are handled

This plugin solved it!

https://www.ads-software.com/plugins/post-expirator/

Glad you have sorted it but ‘Developing with WordPress’ is not really for plugin recommendations but about development.