If found the fix in the last entry here, which links to this: Clear credentials from Workplace Join
In brief, I downloaded and ran WPJCleanUp.cmd, and the work account got deleted.
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I deleted an Azure AD tenant I had created using a personal domain, say "personal.com".
Now, I can't remove the personal account oscar@bugtibaloch .com that managed the deleted AD tenant from Settings > Accounts > Email & accounts (no option to delete it) or from Settings > Accounts > Access work or school (Disconnect button is greyed out).
If found the fix in the last entry here, which links to this: Clear credentials from Workplace Join
In brief, I downloaded and ran WPJCleanUp.cmd, and the work account got deleted.