From the district creations I earlier suspected that the geocode system was started in the early 1980s, and working through the subdistricts a similar timing seems to fit. Though often even the subdistricts created in the 1970s still have their geocode in th chronological order, all those created since the early 1980s are in that order.
There are a few odd cases however which would only be explainable if the system was in fact adopted several years later, and they tried to have the chronological order back for several years. But this wasn't done without mistakes, so the chronological order only fits in most cases.
Maybe the most striking evidence that the system is younger comes from the district Phra Samut Chedi, the southwestern district of Samut Prakan. This district was created in 1984, with five subdistricts split off from Mueang Samut Prakan district (Gazette). All except one of these five subdistrict has an obsolete geocode which it had when they were still part of Mueang district. But not the subdistrict Ban Khlong Suan, which was created in 1979 well before the new district was formed (Gazette). Thus if the geocode system was already in full use in 1984, this subdistrict must have had a geocode starting with 1101. The codes starting with 1101 only have four holes, and there are subdistricts created in Mueang district after the creation of Phra Samut Chedi, so the geocode for Ban Khlong Suan must have been completely forgotten.
Another proof that it was not yet in use in 1983 is the creation of the subdistrict Mueang Ka Rung, Ban Rai, Uthai Thani (Gazette), which has the code 610606, though the number 610607 belongs to a subdistrict Kaen Makrut created in 1917 and 610609 to one created in 1982 - don't know what happened with the 610608, that's one of the missing geocodes.
Another one from the districts: when Khao Kho was split from Lom Sak in 1984 in Phetchabun Province (Gazette), there were no subdistrict geocodes for the five subdistricts while they were still in Lom Sak.
So the DOPA geocodes were introduced after 1984, but for some provinces retrospect geocodes were also created.