Section 80C. When any city or town has conveyed or sold any land under section seventy-nine or section eighty by an instrument in writing conveying or purporting to convey such land, and said instrument is duly recorded in the registry of deeds for the district wherein such land is situated and a period of twenty years elapses after the instrument is accepted for record, and the notice or procedure for the taking and sale or conveyance under this chapter or the instrument or record thereof because of a defect, irregularity, or omission, fails to comply in any respect with any requirement of law relating thereto or the instrument or record thereof shall, notwithstanding such defects, irregularities, or omissions be effective for all purposes to the same extent as though such notice or procedure or the instrument or record thereof had originally not been subject to any such defects, irregularities, or omissions, unless within said period of twenty years a proceeding is commenced on account of such defect, irregularity, or omission and notice thereof is duly recorded in said registry of deeds and indexed and noted on the margin of said instrument of conveyance and in the event of such proceeding, unless relief is thereby in due course granted.