In Asset Store, we can download assets if we have same Unity version or above. However we can't do same, if we use lower Unity version than the asset on Asset Store.
Currently I use Unity 2017 and when I need to download an asset (if it is uploaded with 2018.2) I have to lose time to switch Unity 2018 everytime. Sometimes we have to stick lower versions for our own reasons (such the need to use an old plugin) and needing to update to latest version everytime is very inefficient for our work.
For example now I need to download and try a 3D Model in my Unity Editor quickly, but it is in latest Unity. It is not feasible for me to open other Unity instance that has higher version or to download latest Unity in order to stay up-to-date to Asset Store. I lose precious work time and become inefficient due to this factor.
Usually 2D/3D Models do not contain coding. Therefore it is not necessary to require Supported Unity version. Further, many 2D/3D Artists do not consider users who are probably working in lower Unity version levels too.
I understand, many artists just download latest Unity to upload their models without knowing "supported Unity versions" requirement. But this restricts other Unity users who are dependant to lower versions. And it is not feasible for me everytime to open other Unity instance that has higher version or to download latest Unity in order to stay up-to-date to Asset Store.
* * * SOLUTION * * *
Option1 (Permanent Solution): You can release downloading of Assets (2D/3D Models, Animatons, etc.) with supported-lower-Unity versions, except those that have scripts which can create complication in lower versions.
Option2 (Quick/Temporary Solution): Or at least you can inform Asset Publishers that (in Upload section), if they don't have scripts in their assets, they should consider uploading their assets with lower Unity versions.