is it okay to put a link in the footer

Yes, but do it tastefully. That’s subjective I know but a link to the theme author’s site is allowed in themes here.

If you link to other sites such as customers or businesses then a large amount of bricks will fall on your head and your theme will not be approved.

*Drinks coffee*

Virtual bricks of course. ??

that the user can remove if he/she wants? Or is it not allowed

As long as the user can remove it then that is allowed. Making it so the user cannot remove the link… see large amount of bricks comment.

Is it allowed to sell an upgradeable version of the theme, where the user can get more features? And is it on to have ads for it in the theme?

Thank you so much for the help again, it’s very appreciated ??

It’s when you put links there for other reasons that you can get into trouble. For example: one theme author had put links in his theme footers that he sold. With each update release of his themes users got a new link to “BUY! USED! SOMETHING! HERE!”.

Needless to say that was a longtime ago and that author’s themes are not here anymore. ??

Is it allowed to sell an upgradeable version of the theme, where the user can get more features?

Yes. Many theme author’s host a “lite” version of their theme in the WordPress theme repository here. They include a non-obtrusive link in the theme customizer or settings page with “For more features, please see this site” and link to their site. Also the “lite” version is not crippled in anyway. The “pro” upgrade adds additional features if the user wants them.

As long as it’s not intrusive then that is fine. You can direct users to your site for the “premium” or “pro” version.

And is it on to have ads for it in the theme?

No. (Short answers are often best.)

No theme or plugin in the WordPress repo should have ads in them. Note: an ad is not the same as a link to the author’s site. In the settings of a theme it’s fine to have a link. But having an ad isn’t allowed.

Okay, I explained that very poorly, sorry about that – by ad i ment ad for the upgradeable version

For example:
In the customizer there is a subcategory called “Upgrade to pro” if the user clicks it then there is an image and a description for what they can get if they upgrade, together with a link to a page where they can ask pre-sale questions and read more

Or lets say my theme has a page with documentation and support information, is it then okay to have it in there as well, an image of what features they unlock if the upgrade together with a picture and a link to a sales page, where they can ask questions, see a demo and so on?

Sorry for the long text, english is not my first language but I do my best to explain it ?? Thank you again for the awesome support, its so nice of you