That is not false advertisement, the version you purchased the asset for, per the Unity Purchase Agreements for the Unity Asset store is, a product is sold is sold as is, with the understanding that the product may not be upgraded or further supported. Unity removes unsupported, non-updated assets once they have gone beyond the lifecycle of actively updated Unity versions. The asset is not falsely advertised as, in November, you purchased the CTS version for the version of Unity that was available then, as later versions, such as 2018.3 were still in beta. It wasn't until December 7, Unity 2018.3 was removed from Beta, to which you purchased the asset in November where support for the asset was for Unity 2018.2. Unity 2018.2 to 2018.3 had substantial terrain engine changes that has broken a lot of scripts where a lot of developers have scrambled to fix their assets for that later version, although, they are not obligated to fix it as, again, when an asset is purchased, it is purchased for versions of Unity that are displayed as having been built for, which, CTS was not built specifically for 2018.3 ufntil much later and is patching in support since 2018.3 is the end of the life for version 2018.x. 2019 is the new Unity engine cycle which the update states it will begin supporting fully, specifically beginning with full retro support of 2018.3. I haven't tested CTS 2019 on older versions of Unity but likely legacy support is not available and thus CTS 2019 will only work on 2018.3 and forward as the old CTS will have as much support for 2018.3 that can be made but will be primarily for Unity versions 2017.x-2018.2.