My 54 YO Intellectually or Developmental (IDD) sister-in-law came to live with us in 2017, after her mother could no longer care for her. Parents (for whatever reason) never applied for any type of benefit and she has never received Gov assistance of any kind. We recently tried to qualify her for SSDI, as a (DAC) applicant. Although we have not yet received a denial in writing, we were told by the SSA agent that she will be denied, because of a few years of (SGA) that occurred 25-30 years ago (after age 22.) Because of the SGA technicality, the interviewer said he was unable to accept any medical records evidencing her life long IDD. Of those years with SGA, as well as a few other years where she tried to work and did not meet the SGA, her employment was terminated as a result of her disabilities, the exception being the years when she worked for her mother. (NOTE: She also has bi-polar 1 disorder.) We have some evidence in the form of a letter from a former employer, that reasons in detail why she was let go which seemingly points to her IDD, without specifically saying it. We did not present this to the SSA agent as he seemed determined to deny this claim, based solely on the SGA technicality. Is there even a slight chance that on appeal that the SSA could consider this?
I can't tell you whether or not the evidence you cite would be material, but it certainly wouldn't hurt your sister-in-law's case. When evaluating whether or not a person has performed 'substantial gainful activity' (SGA), Social Security should consider the possibility that the worker didn't fully earn their salary due to special circumstances. When a person is paid more than the true value of their services to an employer, a subsidy can sometimes be established. And, if the person's work would not have been above SGA level without the subsidy, then it's possible that their work could be considered not to have been SGA even if their actual salary exceeded SGA level (https://secure.ssa.gov/apps10/poms.nsf/lnx/0410505010).
SGA can also sometimes be disregarded if the person's work activity can be classified as an unsuccessful work attempt (UWA). To be considered as a UWA, a person's period of SGA must have lasted for less than 6 months and ended due to their impairment (https://secure.ssa.gov/apps10/poms.nsf/lnx/0411010145).
It may be difficult to establish a subsidy or unsuccessful work attempt in your sister-in-law's case, simply because the fact that her SGA level work occurred so long ago makes it hard to obtain any required evidence. The letter from the former employer that you mention may be helpful, but whether or not it would make a material difference in Social Security's determination depends on a number of factors. I can only suggest that you peruse the references from Social Security's operations manual that I've provided links to above, and then submit any evidence or information that you believe is pertinent to the Social Security representative that took your sister-in-law's claim. And, if her claim is ultimately disallowed, you can pursue an appeal of the determination (https://www.ssa.gov/pubs/EN-05-10041.pdf).