After investigating the Adobe 2020 packages that are not installing like AE, ME, PR and the templates provided from the Adobe Enterprise Packages site, they have yet to update the preinstall script included in those packages which checks and restricts it from running on on a higher version of 10.5.x (You can do your own research by using Pacifist or Mothers Ruin Suspicious Package.)

This is the main reason why these packages will fail to install on BigSur.

Same for the InstallationCheck script

Those scripts can be edited/archived/extracted/managed with the following CLI tools on a Linux distribution of your choice:
xar

If you need to edit the installable files, the following dependencies are needed:

xar
gunzip
cpio
gzip
mkbom

For an extensive tutorial on expanding and re-packing please see this article

Additional Note:
Adobe DC 21.0 Installation check fails because ServerVersion is not preset at

In BigSur, that path does not exists.

Hopefully, adobe fixes this in the future....not so far way from now...