When you create an SSIS package you specify the version of SSIS to run against it. If you're using VS 2015 then that means you're using the unsupported SSDT for VS 2015 version. SSDT was lockstepped with the SSIS version that shipped at the same time. The older versions will target an older version of SSIS however because it interacts with SSIS via COM and newer SSIS versions ship the older COM versions your SSIS package may run just fine on the newer SSIS server if you copy them to it. You may run into issues if you try to use the designer to build and run against a newer SSIS version though. You also won't be able to take advantage of any of the newer features.
My personal recommendation is to install Visual Studio Community or use a current version of VS 2019. Then install the SSIS extension for VS 2019. Now you can use VS 2019 to build SSIS packages that run against SQL 2017 or SQL 2019.