What is the longest observable wavelength of light using an optical telescope? How far outside the visible spectrum could an optical telescope feasibly operate?
I guess it depends what you mean by an optical telescope.

However, if you just mean any telescope that can record images at visible light wavelengths, then it is possible to use these (with the appropriately cooled instruments) to make observation at so-called mid-infrared wavelengths of $\sim 20$ $\mu$m.

See for example on the Gemini telescopes
https://www.gemini.edu/sciops/instruments/midir-resources

The background noise at such wavelengths is extremely high - the telescope and sky look hot at these wavelengths - so these instruments have largely been superseded by space observatories like Spitzer and WISE.

Almost all large "optical" telescopes are routinely used to make observations out to $2.4\ \mu$m (the K band).