Hi Partner,
Inactive contacts won't display in Account's CONTACTS subgrid, that's OOB design.
However, due to association between them still exist, we can retrieve all associated contacts under a specific account even if some of contacts were inactive,
and distinct active contacts and inactive contacts by contact's statecode field.(0 stands for active, 1 stands for inactive, see reference about contact entity)
Thus you could insert a web resource to Account form, create a custom HTML table from retrieved data and give special text color for inactive contact records.
There is a helpful tool to generate retrieving code quickly:
https://github.com/jlattimer/CRMRESTBuilder
Choose contact_customer_accounts relationship.(account to contact, 1:N)

All related contacts will be expanded in JSON result.

Regards,
Clofly