VCC is your logic voltage

The pin sees VCC

When the slot is blocked, the comparator lets go of SIGNAL and R7 pulls it up to VCC. There is nothing else in the way, so HIGH on SIGNAL is whatever voltage you wired VCC to. LOW is the comparator's 0.15 V, whatever VCC is.

Pick your board and try both supplies:

• ESP32, ESP32-S3, Pico: 3V3. Their pins run at 3.3 V and read HIGH from 2.5 V (ESP32 family) or 2.0 V (RP2040). 3.3 V on SIGNAL is right.
• Uno: 5V. Its pins run at 5 V and need 3.0 V to be sure of HIGH. 3.3 V scrapes in; 5 V is right.
• 5V beside a 3.3 V board lifts SIGNAL toward 5 V. The pin's protection diode catches it and about 140 µA flows into the chip through it, every time something passes the slot. The 10 kΩ keeps that small, but it is outside the pin's ratings, and on a Pico VBUS is the same mistake.

The block does not mind

The LM393 runs from 2 V upward and the slot sensor's LED only needs a little over 1 V, so the block works from 3.3 V as well as from 5 V. At 3.3 V the infrared LED gets about 2.2 mA instead of 3.9 mA, and the light sensor passes less with it; a beam across a slot says what that means for the margin.