Foolish me, I left auto-updates of firmware turned on. Two nights ago, two of my TP15 smart plugs updated to 1.4.2, and home assistant stopped working with them. I tried with python-kasa command line, and I get "Authentication failed for device"
I had one spare plug that was offline that is still on 1.2.4, and it is still working with python-kasa

Failing plug:
== Authentication failed for device ==
== Discovery Result ==
Device Type: SMART.TAPOPLUG
Device Model: TP15(US)
IP: 192.168.86.71
MAC: EC-75-0C-2B-75-29
Device Id (hash): redacted
Owner (hash): redacted
Supports IOT Cloud: True
OBD Src: tplink
Encrypt Type: AES
HTTP Port: 80
Login version: 2

Thoughts?

Oh, quick edit. I did do a full factory reset of one of the plugs and re-configured it. No difference.