In general, when one page is linking to yours – it is passing its link juice – meaning – it gives more credibility to your site and signals to search engines that they value your website. In essence, you can consider that as an endorsement. If a high authority webpage links to you...
The reason why the css-class rel-something was not moved to the rel--attribute is that the value something never has any standardized meaning and therefor is not included in the list of allowed values. A list of allowed values can be seen here: https://developer.mozilla.org/en-US/docs...
In this case, the code removes the noreferrer value from new or updated links, meaning they will be the only ones affected by this change. You can also remove the noreferrer value from existing links if you need to. That can be done manually, using the Classic Editor, or automatically, ...