I read somewhere that you should not mark bad sectors as bad, if the number of bad sectors is growing. Is this true?

I do not know what you read but that is nuts bad is bad. Replace the hard drive as soon as you can it is failing.

Here is a description of what a bad sector is.

A bad sector in computing refers to a disk sector on a disk storage unit that is permanently damaged. Upon taking damage, all information stored on that sector is lost. When a bad sector is found and marked, the operating system skips it in the future.

This is the reference of the description.

https://en.wikipedia.org/wiki/Bad_sector#:~:text=Bad%20sector%20in%20computing%20refers,skips%20it%20in%20the%20future.

If the bad sectors are increasing fast then it needs to be replaced. Back up your data your HDD is failing. Also, 8960 bad sectors is a big number I would replace it. If under warranty give it in they replace it for you. Hope this helps anyone with the same issue. :)