I only mention the @username feature as an example. There have been other recent changes, such as the automatic recognition of image files and HTML links.
Are these things being implemented by selecting options available within the SMF program - in the same way that I can select different Preferences in my Profile?
Or are these changes being implemented by modifying the SMF source code?
If it's the latter then IMHO it is sheer madness, and no wonder that there are problems.
...R
The answer is all of the above.
In many cases a feature or function may already be present in some form as was the case for @username.
Most often it is a matter of checking that it still works as it was usually turned off a very long time ago and the reason for that may not be clear anymore.
Hence the reason for testing elsewhere. Although that does not always show a bug as was the case with @username.
In some cases additional code needs to be implemented to work through newer additions such as SSO or AUTH0.
Those can put up additional issues all by themselves due to the age of the SMF software in the grand scheme of things and its inherent incompatibility with newer security.