It depends on what type of tracking you are looking to do, and the relationship type being used by the subgrid. If this is a One-to-Many relationship, then adding a record to the subgrid is easy to verify as there is now a record in the child table. Removing is harder to track, provided the user is deleting the record, as there is no history directly in CRM once deleted.
Many to Many relationships are harder, since the records they create are much more hidden and difficult to find.
Depending on what you need to track or so, you could look at creating a custom audit table for the records/relationships in question. When a record is added, you can use a real-time workflow to create a new audit record. Similarly, you can use a real-time workflow prior to the delete action to capture this change as well. This would work fine for the One to Many situation.
The Many to Many tables can't be access by D365 workflows, so you are stuck to using Power Automate or plugins to capture the actions being taken. The same concept applies, however, in that you can trigger a flow or plugin step to run on create or delete of the relationship record for auditing purposes.
Above is an older, but good blog, discussing how Many to Many relationships work in CRM and how you can query/access the records.
