First I tried the recommended method from repository …
wget https://repo.percona.com/apt/percona-release_latest.$(lsb_release -sc)_all.deb
--2021-08-12 09:01:48-- https://repo.percona.com/apt/percona-release_latest.hirsute_all.deb
Resolving repo.percona.com (repo.percona.com)... 198.211.122.205, 167.71.73.251
Connecting to repo.percona.com (repo.percona.com)|198.211.122.205|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2021-08-12 09:01:49 ERROR 404: Not Found.
Next I followed the instructions to download the package …
sudo dpkg -i ./percona-xtrabackup-80_8.0.4-1.stretch_amd64.deb
Selecting previously unselected package percona-xtrabackup-80.
(Reading database ... 198357 files and directories currently installed.)
Preparing to unpack .../percona-xtrabackup-80_8.0.4-1.stretch_amd64.deb ...
Unpacking percona-xtrabackup-80 (8.0.4-1.stretch) ...
dpkg: dependency problems prevent configuration of percona-xtrabackup-80:
percona-xtrabackup-80 depends on libcurl3 (>= 7.16.3); however:
Package libcurl3 is not installed.
dpkg: error processing package percona-xtrabackup-80 (--install):
E: Package 'libcurl3' has no installation candidate