I think "taxable" for your "tax-liable" is a good choice, although "listed" might be an idea for a still more concise translation. On the other hand, since the слободы were generally areas exempt from the taxes imposed on тяглые крестьяне and some others, probably "listed" is not enough.

You could write it like this:

"The 1639 decree On the Prohibition of the Sale of Taxable Households [Properties?] within Settlements [within Sloboda Settlements?] to Tax-Exempt Persons mentions..."

Here's what Pushkarev's "Dictionary of Russian Historical Terms" says:
"tiaglo, tiagla or tiagly. Burden, a term which came into use during the era of Mongol-Tatar domination. In Muscovy, the term tiaglo designated the sum total of fiscal obligations and labor and service duties imposed by the government on the listed members of a rural or urban community. The term tiaglo designated also the capacity of the taxpayers to meet their obligations, based on their labor force and taxable objects, such as lands and businesses."