Merge conflicts come with helpful characters ("<<<<<<<<") to show where the conflict originated. If you merge two branches (especially if they are large or reviewed on separate days), its pretty easy to overlook the fact that they share an error code. This will not raise a conflict in git. But is in fact a merge error and users will experience a regression because of it.

> Including a unique code with an error message does help with SEO when users search for a solution.

Maybe. But I don't see how Googling "Product name 40001 error" is better than the user reading "account_deactivated", understanding intuitively what happened and Googling "Product name reactivate account" or "Product name account deactivated".

It sometimes depends if you are relying on first party documentation or not.

I have used systems with full and proper documentation and searching for an error code will give an exact match and as much detail as I want, sometimes in offline documentation and sometimes online. While error messages often cause problems on Google.

For your example, try selecting it and searching 'account_deactivated' in Google.

Showing results for account deactivated Search instead for account_deactivated

Click that 'search instead' link. Google clearly ignores the underscore in the results so it's not doing what it says it will. The way around this is to quote the text but IMO searching something like '<productname> error 0x5678' is clear, reliable, and easy. These will often be paired with a token like your example anyway, such as IRQL_NOT_LESS_OR_EQUAL, so you get both.