Why is not jthread::get_stop_source const?

The question says it. [thread.jthread.stop]/1 says:

[[nodiscard]] stop_source get_stop_source() noexcept;

Effects: Equivalent to: return ssource;

Why is it not a pure observer?

Answer

A stop_source object allows you to request that the thread which has such an object (or its attendant stop_token) perform a stop. Such a request is not logically const. As such, if you fetch a stop_source for a jthread, it is expected that you are going to perform the aforementioned “not logically const” operation.

So the function you used to retrieve it is not const. Note that getting a stop_token is const, as this is an observer of thread-safe state, not a modifier of it.