Kentico 8.2 and 10 compatible with Windows Server 2018

You're mentioning WIndows Server 2018. Are you talking about the server operating system or the SQL Server database version? I don't believe there is a Windows Server 2018; there is 2014, 2016 and 2019 though. There is SQL Server 2018 however.

You may want to clarify a bit more what you mean. Also, it doesn't hurt to take setup the server and test the sites out on your own. You can ask Kentico for development domain alias licenses.

You can see the officially supported web servers and SQL servers in the documentation system requirements for Kentico 8.2 and Kentico 10. The mentioned systems were tested with given Kentico version. Other versions of the systems were not tested so they are not officially supported, although there should not be any problem as soon as you ensure .Net and IIS versions and when it comes to SQL Server, you can still restore the DBs in a compatibility mode.

It is Window Server 2019.

From an OS perspective, Server 2019 should work as long as you can get the .NET framework installed to run the sites. As I mentioned, set the server up and add the sites and give them a test. Keeping in mind, you won't get any official support if something on the servers OS is causing a problem though.