The Oscar-7 amateur satellite, aka AO-7, was launched into Earth orbit on November 15, 1974 and included an automatic telemetry downlink by radio.
After several years' operation, the batteries shorted out and silenced the satellite. In mid-2002, however, it was suddenly heard again, after the batteries went open-circuit and allowed the satellite to run purely on solar power. Since then, it has continued to work quite well, with everything besides the batteries working essentially as well as it did circa 1980.
As far as I can tell, the telemetry only reports the values of certain measurements taken on board, and does not provide any error flags or interpretation of these values. The format of the data is in groups of figures transmitted using Morse code, which can be received and decoded by a reasonably modern computer with an attached radio receiver:
100 176 164 178
280 262 200 254
375 358 331 354
453 454 461 459
541 501 552 529
600 600 601 651
The last group of digits in the block, reading 651, is the one which tells you whether the telemetry itself is reliable. Ideally it should be 650, but 651 is close enough. The telemetry is sent continuously, delimited by the "HI HI" callsign markers, which possibly counts as "periodically" given the time it takes to send a single block.
Also predating 2000 is Oscar-27, aka AO-27, which was launched on September 26, 1993. Being almost 20 years newer than Oscar-7, telemetry is provided in a directly computer-readable format (1200-baud AFSK).
AO-27 is a secondary amateur communications payload carried aboard the EYESAT-1 experimental MICROSAT satellite built by Interferometrics Inc. of Chantilly, Virginia. The commercial side of the spacecraft's mission is the experimental monitoring of mobile industrial equipment.
Sample of raw telemetry data from Oscar-27:
[TIMESTAMP] OS 117 [000/11:18:30] [Sat Feb 28 20:30:17 2009]
[HWS] 1235853007 <BCR Setpoint 1F><TXC Exciter ON><TXC 5v HPA ON><Rx=082F80><Modem=88FFE4><BCR=180D1F><Tx=000377>
[SWS] <TXMODE=2> <WDOG1=80><WDOG2=0><WDOG3=0>
[SWS] <TEPR1=0, 0><TEPR2=0, 0><TEPR3=0, 0><TEPR4=0, 0><TEPR5=0, 0><TEPR6=0, 0>
Some of that data is more-or-less human-readable. The rest is also self-describing, in that each data element is unambiguously delimited and associated with a unique field name, and can thus be parsed by a computer without prior knowledge of which or even how many elements exist.
Telemetry data is usually sent on a schedule timed to coincide with passes over the Northern Hemisphere, thus periodically, and is followed by a period of payload operation.
Oscar-27 also monitors its own battery charge status, and will automatically shut down the more powerful payload transmitter when the batteries run low (which can happen during "eclipse season"), switching to a mode where it sends telemetry instead using a weaker transmitter. Thus it effectively also sends telemetry in response to an error condition.