This is a very simple question.
Is phpseclib 1.0.18 compatible with PHP 7.3?
I see no answer to this on their website, nor can I find any specific reference to PHP7.3 in this support site.
According to the 1.0 changelog, the 1.0.17 release included fixes for PHP 7.4:
Additionally, the Travis CI log shows a successful build against the 1.0 branch and PHP 7.3
So I would say yes, PHP 7.3 and the 1.0.18 branch are compatible.