It all depends on the kind of license you have. To check, click About under the question mark in the upper right corner and then Manage license. If it reads 'user license' then you are on subscription. If it reads something like 'perpetual' you have a lifetime license. If you are on subscription, then you have to pay Autodesk a fee on a regular basis. If you don't pay, you're not allowed to use the product anymore. If you have a valid subscription, then there's something called 'previous versions' which, if granted, means you can use a version not covered by the license. But if you are on subscription, I wouldn't go that way but instead install a newer version.
If you have a perpetual license, then you're free to use it 'as-is' as long as you like, but with new versions of Windows emerging, sooner or later it won't run anymore. Also, version 2018 is no longer supported which basically means there will be no updates to address problems.