ATtiny85 MIDI IN with less than 3V -can it be done?
The H11L1's minimum supply voltage is 3&nbsp;V.
There are low-power optocouplers like the TLP2361, but event those do not go below 2.7&nbsp;V. (And those limits are actually designed for 3.3&nbsp;V&nbsp;±&nbsp;10&nbsp;% or 3.0&nbsp;V&nbsp;±&nbsp;10&nbsp;%.)

To get a high-speed optocoupler for lower voltages, you have to build it yourself from discrete parts, e.g.:


(R1 is sized for the PC817-A's minimum CTR of 50 %; higher CTRs should use a lower value for R1. Q1 can be any random small-signal transistor. Adjust R2 for about 1 mA, depending on VCC.)