Yes, but as a programmer if you had the source code and the inclination you could still make modifications to suit your needs.

I was a programmer at that time and I was even reasonably well-versed in the rather arcane dialect of C++ that was used in Symbian programming.

Making a non-trivial change to an OS and deploying it successfully to a device on my own would be a challenge, though. Once Nokia stopped paying the internal developers and Symbian programming forum was closed, there were no gurus capable of answering questions.

And in case of a very complicated OS whose internal workings were very different from other OSes out there, having no vibrant community of experts = death, unfortunately.