on Ubuntu 20.04 you need to install myspell-de-de / hunspell-de-de using:
sudo apt install myspell-de-de --yes (installs hunspell-de-de as well)
It would also be nice to be able to use my custom dictionary from Libreoffice 6.4 in Libreoffice 7.
This should be available automatically from your previous LibreOffice user profile unless you deleted your previous user profile.
Libreoffice 7 is still being tested. I will stick with Libreoffice 6.4 for now and hope that dictionaries will become easier to use in a future release of Libreoffice 7.
The issue got absolutely nothing to do with the version of LibreOffce, but the way how Ubuntu maintainers of LibreOffice package the software for their distribution. If you want that to change, you need to turn to maintainers of Ubuntu distribution packages unless you decide to use
.deb packages from TDF (libreoffice,org) directly in the future (though official statements do not recommend to do so). Using Ubuntu packages requires (and has required in the past) to install myspell / hunspell packages. So it won’t get easier, it had not been easier in the past and sticking with 6.4. won’t provide a path out of this situation.
I have been using Libreoffice 6.4 for some time and it is still installed.
My concern is, that having installed and using version 6.4 and 7.0 at the same time, may have corrupted your LibreOffice user profile in
$HOME/.config/libreoffice/4 and: Consistently using Ubuntu repositories disables the option to have both versions to be installed at the same time. Ubuntu 20.04 provides 184.108.40.206 and using LibreOffice PPA for fresh would override/replace 6.4.6. Hence you must have installed 7.0.x (whatever
x is) from some other repository (which you did not mention). In that sense, from a distance, your problem with German dictionary appears to be a bit self-inflicted.
Hope that helps.