The 3D Warehouse supports the latest three versions of SketchUp. End of support for SU2018 and SU2017 was announced several months ago and ended in the last week or so. This end of support was delayed a year for SU2017.
It is possible to get components by using the Components panel instead of the 3D Warehouse interface. Alternatively they can go to the 3D Warehouse via their Internet browser, download the Collada file, and import the included .dae file into SketchUp.