Great question and I am going to be honest I am not sure of the answer. I would suggest emailing firstname.lastname@example.org and they should forward to product team members that might be able to give you more insight.
My gut feel would be probably no support for .NET 5 in Kentico 9 as its already in full development cycle. It may be compatible but doubtful it will take advantage of anything new at this point. There will be a lot of further support for MVC and other current trending technologies though.
With that being said, Kentico has a good reputation of integrating new stuff much faster than others, so that doesn't necessarily mean we will have to wait until 10 to take advantage. They update so regularly I would say there is a decent chance support would come earlier.
If you learn anything more, please share with the group.