unrar-nonfree can be installed from Debian armhf repository (it's simply called unrar there). For example, in Raspbian Buster you can run:
wget -qO - https://ftp-master.debian.org/keys/archive-key-10.asc | sudo apt-key add -
echo deb http://deb.debian.org/debian buster main contrib non-free | sudo tee -a /etc/apt/sources.list
sudo apt-get update
Thanks to @Jaromanda X I now know that the problem was due to wrong apt setup.
To be precise, some libraries still do not support buster and in case of Snips (for offline speech detection), the suggested workaround was to switch to stretch repository for it (namely, https://raspbian.snips.ai/stretch).
After doing so, while trying to install libatlas-base-...
This worked for me:
sudo rm /var/cache/apt/archives/wpasupplicant_2%3a2.7+git20190128+0c1e29f-6+deb10u1_armhf.deb
apt-get upgrade -f
Credit to: https://askubuntu.com/questions/1011008/dpkg-error-cannot-copy-extracted-data-for-unexpected-end-of-file-or-stream