Hello,
Thanks for contacting us and sorry for any inconvenience that might have been caused due to that.
We already have an option that allows you to exclude posts/pages from the sitemap using the id of the posts. Please navigate to WP Dashboard > Rank Math > Sitemap Settings > General and add the ids of the posts you want to exclude in the Exclude Posts option:

If you are not able to see that option then you must be using the easy mode of Rank Math. Please switch to Advanced Mode using this guide: https://rankmath.com/kb/advanced-mode/
And here is how you can get the post ids: https://mythemeshop.com/blog/how-to-find-post-category-tag-comments-or-user-id-in-wordpress/
Hope this helps. Let us know if you need any other assistance.
Hello,
We are super happy that we could address your concern. If you have any other questions in the future, know that we are here to help you.
If you don’t mind us asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.