Is this communication I²C?

Given that there are only 8 clocks per byte (I2C requires a 9th clock for the ACK/NAK bit) and the clock idle state seems to be low, I would say that this is more likely a SPI (or SPI-like) interface.

Not sure about the extra clock width on the first bit of each byte, however.

My guess is that it's some company's homegrown "I2C-like" protocol. There were some of those back in the day when using I2C meant having to give money to Philips.

It appears to have an ACK (the short pulse on the data line prior to the clock stretch looks a lot like the data line getting passed from master to slave).

Oddly, it appears to transmit 7 bits at a time.

I'll toss my hat into the ring...

If these are old devices you could be looking at some "bare minimum" 7-bit synchronous RS-232 variant:

  • That longer pulse in the beginning of each frame could be a start bit, and

  • The plateau in the clock signal at the very beginning could be return to 0 before going to negative "mark". (You did not provide voltage on screenshots, so I am guessing here).