CRA generally requires full source deductions on LTD payments made to a former Canadian (now U.S.-resident) employee notwithstanding 15% Treaty limit

Long-term disability payments made by a Canadian employer to an employee who, after becoming disabled, became a U.S. resident, would be considered by CRA to be remuneration ("because they would be amounts arising out of the employment relationship") and therefore usually would be subject to regular source deductions under Reg. 102.

However, the term the term "pensions" is defined in Art. XVIII of the Treaty to include "an amount paid under a sickness, accident or disability plan." Accordingly, the U.S. resident  "could file a Canadian income tax return in order to obtain a refund of any withholdings made in excess of the 15% amount specified in paragraph 2 of Article XVIII."

Neal Armstrong.  Summaries of 26 September 2014 T.I. 2014-0531441E5 under s. 115(2)(c) and Treaties – Art. 18.