That is the date the law changed so that you can use the $250,000/$500,000 exclusion on the sale of your Principal Residence.
Depreciation before that date is eligible to be covered by the $250,000/$500,000 exclusion. Depreciation after that date is not eligible to be covered by that provision.
I can't answer this with any degree of certainty. I also think that there's something about depreciation recapture on property placed in service before 1987 also. But don't know any of the particulars on that. A preliminary search of Pub 523 and 527 doesn't cause anything to "jump out" at me either.
That is the date the law changed so that you can use the $250,000/$500,000 exclusion on the sale of your Principal Residence.
Depreciation before that date is eligible to be covered by the $250,000/$500,000 exclusion. Depreciation after that date is not eligible to be covered by that provision.