License Key Compatibity (Backwards Compatibility)

No, it is not. So you're goal is to keep running cf2018 and NOT yet move to cf2021 for some reason, right?

First, note that what you're wanting to do is known formally by Adobe as "backwards licensing" (buying the currently available version--all that's possible--but wanting to get a license instead for the prior version). And whether you can do it after buying the new license (you can't, always) is discussed in a blog post by Adobe several years ago here:

https://coldfusion.adobe.com/2016/03/adobe-coldfusion-backward-licensing/

Irs very important that you read the comments as well. 

Second, do beware that whatever your motivation is to remain on cf2018, its end of life is coming up in July 2023. It will get no more updates, including security updates. This is documented here:

1. https://helpx.adobe.com/support/programs/eol-matrix.html

And beware taking any consolation in the available "Extended support" being to 2024: that's not for extending the date to get updates, it's simply paying for support to help with migration. For more, see this post from Adobe discussing it in the context of when cf11 support was ending in 2019:

https://coldfusion.adobe.com/2018/12/adobe-coldfusion-support-policies-and-options-faq/