For the time being you can manually download and install the latest version of ZeroTier One for Debian Bullseye here: Index of /RELEASES/1.6.5/dist/debian/bullseye/
Then you can manually add the official ZeroTier mirror to get future updates:
sudo apt install apt-transport-https ca-certificates curl gnupg lsb-release
curl -fsSL https://raw.githubusercontent.com/zerotier/ZeroTierOne/master/doc/contact%40zerotier.com.gpg | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/zerotier.gpg
echo "deb https://download.zerotier.com/debian/$(lsb_release -cs)/ $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/zerotier.list