On the CPTDB Wiki OC Transpo page, the information on when any high-floor buses were retired is still far from complete.
With that being said, I will give further information I was able to gather for when some were retired.
1989
7128 (GMC TDH-5305, built in 1971).
1990
7132 (GMC TDH-5305, built in 1971); 8801-8806 (Orion I, built in 1978); 8807-8824 (Orion I, built in 1979); 8825-8832 (Orion I, built in 1980).
1993
7284 (GMC-5307N, built in 1972); 7344, 7350 and 7357 (GMC-5307N, built in 1973); 8709 and 8717 (Flyer D800B, built in 1978); 8242-8243, 8245, 8247-8251, 8253, 8258-8259, 8261 and 8264-8266 (Orion I, built in 1982); 8373, 8375 and 8384 (Orion I, built in 1983); 8745-8746 (Orion-Ikarus III, built in 1987).
1998
7409 and 7414 (GMC-5307N, built in 1974).
1999
7455 and 7459 (GMC-5307N, built in 1974); 5705-5708, 5716 and 5754 (GMC-5307N, built in 1975); 7802 and 7831 (GMC-5307N, built in 1978); 7904, 7913-7914 and 7924 (GMC-5307N, built in 1979).