If you run "apt-get install lsphp73-pear", did it show it has been installed successfully or failed?
What this command returns: "dkpg -l | grep lsphp73", does it include lsphp73-pear.
Install was successful.
ii lsphp73 7.3.22-1+stretch amd64 server-side, HTML-embedded scripting language (LSAPI binary)
ii lsphp73-common 7.3.22-1+stretch all Common files for packages built from the PHP source
ii lsphp73-curl 7.3.22-1+stretch amd64 CURL module for PHP
ii lsphp73-dev 7.3.22-1+stretch amd64 Files for PHP70 module development
ii lsphp73-igbinary 3.0.1-1+stretch amd64 igbinary PHP serializer
ii lsphp73-imagick 3.4.4-1+stretch amd64 Provides a wrapper to the ImageMagick library
ii lsphp73-imap 7.3.22-1+stretch amd64 IMAP module for PHP
ii lsphp73-intl 7.3.22-1+stretch amd64 Internationalisation module for PHP
ii lsphp73-json 7.3.22-1+stretch amd64 JSON module for PHP
ii lsphp73-memcached 3.1.5-1+stretch amd64 memcached extension module for PHP, uses libmemcached
ii lsphp73-msgpack 2.0.3-1+stretch amd64 PHP extension for interfacing with MessagePack
ii lsphp73-mysql 7.3.22-1+stretch amd64 MySQL module for PHP
ii lsphp73-opcache 7.3.22-1+stretch amd64 Zend OpCache module for PHP
ii lsphp73-pear 1.10.10-1+stretch all Provides PHP PEAR package
I tried switching to 7.4 and got the same result.