So I could always continue using my Lazarus IDE v1.3 executable and just change the compiler setting to point to FPC 3.0
To be precise, version 1.6 of Lazarus IDE and LCL still supports Win9x and Win2k and "fixes_1_6" branch will always support them.
The last release will be 1.6.x (maybe 1.6.4 or 1.6.6) somewhere in late 2016 or early 2017. That version can be used forever by people who need it.
However FPC 3.0 dropped the support from its libraries. The compiler itself may work on Win2k but not all user code. That's what I understood anyway.
It means you may have to use FPC 2.6.4 together with the mentioned Lazarus versions.