Drupal 8.x

I developed a small drupal module which bring the external Js to head tags.

I'm not new to Drupal but all my sites are Drupal 7 and I started migrating them to Drupal 8. I'm trying to get used to the current work flow, and I'm starting to use configurations imports and exports. I understand that with this we can move from development site to production site without having to touch directly the database. I already have development and production in sync, and I did some work in the development site such as installing a couple fo modules, and changing a couple of configurations. One of the new modules is pathauto and I bulk converted paths for a lot of content.

I have some scripts that I need to run manually, these scripts are not meant to be ran on a regular schedule, therefore, I DO NOT want to use cron. With that, how can I kick off a php script in the background?

I've tried to use `exec` but it doesn't know what drush is, so I can't run my script that needs ran via drush.
I looked into the `Background Process` module but 1) it hasn't been updated in years and 2) it doesn't have a Drupal 8 version. 

How can I run a script in the background that isn't done through cron?

I see that in D7 tracker module was using format_interval call to render time intervals with "ago".

Now in D9 I see that it is using formatTimeDiffSince method on DateFormatter class (that in turn calls formatDiff method.

My question is what is the reason not to use formatInterval method on the same class. (which is like D7 format_interval function). I do not see any module using formatInterval method anymore. (Correct me if wrong)