>whether the transaction has tax relevance,
It is almost always a matter of the account having tax relevance, not each individual transaction that involves that account.
> the only way to represent that expense in strict double book is to have 2 accounts for (Repairs_Tax, Repairs_Notax)
There are very few accounts that need to be split like that for tax purposes. Meals & entertainment are one such (although the tax laws about that change from year to year sometimes), and maybe the part of dues to a professional organization that are for political lobbying purposes. It's really not a problem at least in the U.S. tax code.
And note, as I previously commented, it is quite common for a business to have at minimum two sets of books, one for tax and one for book (the company's own internal representation of its financial position).
>[T-accounts] just seem to be a visual method to represent negative numbers
T-accounts are an instructional tool, they are not used for actual production accounting purposes. And they don't represent negative numbers, they illustrate a journal entry's effect on the general ledger accounts. Therefore, there is no need to include a date attribute. Rest assured, in a real double-entry system, every single debit and credit is date-stamped.