Post Numbers is retired. It no longer gets updates or fixes.
It added a #N number to each post in a discussion, as a Flarum 2 port of ziven/flarum-post-number. @clarkwinkelmann's Prominent Post Numbers now does the same job on Flarum 2 and more: post numbers in mentions, a custom prefix, and a setting to show the number on the right. So I retired mine in favor of it.
If you still have it installed, it keeps working. To switch:
composer remove linkrobins/post-num
composer require clarkwinkelmann/flarum-ext-prominent-post-numbers
php flarum cache:clear
Prominent Post Numbers: https://github.com/clarkwinkelmann/flarum-ext-prominent-post-numbers
