Do you have access to Office 365 PowerShell? If so this post from Aaron Labiosa might be helpful.

How to purge items from a large history list safely via PowerShell 

I do have access to Office 365 Powershell but this script is written for onprem and does not really work for online and I'm not entirely sure how to modify - does anyone know how?

Hi Jonathan,

Over what time period were these history items created? I believe by default SPO should automatically purge items over 60 days old by itself.

Thanks,

Callum

I can't tell because of list view thresholds but presumably they go back some time given there were over 8 million records.  I've set a retention policy to see if that helps but after a week (it's set to permanently delete items created over 7 days) nothing has happened.

Jonathan Kite‌ with on-prem there is a link in the Nintex Workflow section of Site Settings for purging workflow data. I assume that option is not available with O365?

There is no Nintex Workflow section on O365 unfortunately.  

Hi Jonathan,

It may be worth raising a support case via the O365 portal for this, I believe this should naturally be handled by the SPO platform for you automatically.

add configurations for enabling/disabling Workflow Auto Cleanup and Auto Cleanup Days – SharePoint Experiences in Office… 

Thanks

Callum

Hi Jonathan,

Were you able to find a solution for this?

Callum

Unfortunately nothing graceful

Did anyone find a solution for this?

Hi Anna,

I would recommend contacting Microsoft support for SharePoint Online with this question, it should be handled automatically by SPO:

The history purge job is not something that Nintex has control of in SharePoint Online/365

Do your workflow instances show this label like above?

Callum