For its 2021-22 season, the Columbus Symphony will celebrate two momentous occasions: first, the 70th anniversary of the organization’s founding in 1951; and second, its emergence from the current, pandemic-altered season. Although a reduced version of the symphony has been giving a steady stream of limited-capacity concerts in the Ohio Theatre this season, symphony leaders acknowledge that audiences expect big and bold concerts in the season ahead. “We were going to do the very best we could in every circumstance, but we also have been saying for a good six months that we were not going to limp out of COVID — that we were going to burst out of COVID,” Executive Director Denise Rehg said. “That is what the season is meant to do.”