As I am sure you already know, Canadians planning pursuing residency/fellowship training in the United States on a J1 visa are required to obtain a Statement of Need from Health Canada. Health Canada creates a List of Needed Specialties each year, if the specialty you are intending to pursue is not on that list, you will likely not be able to obtain a Statement of Need, although there have been exceptions made in the past.
The most recent List of Needed Specialties has stated that those who started their residency in 2015 or prior will be able to obtain Statements of Need almost every subspecialty. So if you have are already in residency, you will likely be unaffected by any changes to the List of Needed Specialties.
If you will be starting residency from this year onwards, there have been some recent changes that may affect you. Health Canada has stated that Statements of Need for subspecialty training will only be issued for the purposes of meeting "Specialty Training Requirements". For example: 4 years of ACGME accredited training in Internal Medicine are required to obtain certification from the Royal College of Physicians & Surgeons in Canada. As you know, Internal Medicine training in the US is 3 years long. So Health Canada will issue Statements of Need for 1 year fellowships like Geriatrics, Sleep medicine, Palliative care, and Chief resident positions which will allow you to fulfill the requirements for certification from the RCPSC. Health Canada will only issue Statements of Need for other subspecialty training if you are fully licensed in Canada and certified by the RCPSC. Basically if you want to become a oncologist/intensivist/cardiologist etc., you will be required to complete 3 years of Internal Medicine, a 1 year fellowship/chief resident year, become fully licensed in Canada, and then pursue your intended specialty.
The good thing is that for Internal Medicine fellowships, these changes will only affect residents who will be completing their training in 2020 or onwards. Those who have started their residency already are safe.
More information here:
http://www.hc-sc.gc.ca/hcs-sss/hhr-rhs/postgrad-postdoc/cat_b-list-liste-eng.php