Vcc and Gnd supply the power, are hardwired to the silicon.
Leaving six configurable pins. Now that we have an e-switch, switch LED, three LED driver banks, and possibly an Aux. LED board to control (though that can run autonomously from its own MCU in Lexel’s proprietary design), we have run out.
Some pins are reconfigured during flashing, which has to be considered, so anything attached to them doesn’t interfere with the process.
The ATiny85 is now a very old design (2005 introduction).
Since then, Microchip took over Atmel (2016) and have developed the platform, also introducing technology from their PIC designs. Tiny leadless SMD packages allow much higher pin counts (though are more challenging for DIY assembly.)
This is where future development will have to go, and from where I would be starting, as a new developer.
Legacy code bases can be ported, and design environments updated. Already happening, e.g. see Adventures in TinyAVR 1-Series