Most IoT devices support 2.4GHz only. Notably this applies to ESP32-based devices and older phones and laptops too. I would argue that it is the 5GHz band that is optional, the only benefit (bandwidth), being relevant only for laptops and phones when downloading something.

Let me guess you don’t live in an apartment?

5ghz is important if you are interference limited. The lack of wall penetration and short range become benefits.

2.4ghz provides very poor performance in my condo due to the 30 different SSIDs I can see from my lounge room.

The 30 SSIDs your device can see are bad, but what might be even worse are the many non-802.11 devices on 2.4 GHz that are invisible to a simple SSID scanner and don't share bandwidth fairly with 802.11 CSMA/CA. (This includes Bluetooth!)

5 GHz has much less of that, but big parts of it have weather radars as a primary user, with APs being required to detect and avoid any channel where they can detect one.

It penetrates walls so much better than 5GHz. If I lived closer to my neighbors I would agree with you.

Plus I still have some esp8266-based widgets that I made which only operate on 2.4GHz.

If you don't need it, of course, you might as well deactivate it. But if you do, I don't see the point of having two different SSIDs if you don't need them for another reason anyway.

I do need it, but the IoT devices are conveniently close to most APs, so that sort of evens out.