What does the author mean with “legacy”? I thought that meant “abandoned”.

Legacy to me does not mean abandoned, but the previous version that is still needed. It does not tell you if its "supported". Abandoned would be a software no longer in "supported" to me. But that does not say if its still needed today. So legacy and abandoned are similar, but not the same, only sometimes the same. Legacy software or hardware can be popular in usage too. In example old graphics cards like GTX 1070 are legacy and use legacy drivers. They are somewhat popular still. The official drivers from Nvidia still support this older graphics card, so they are not abandoned, only legacy.

This is what my definition of these words. I don't think Github itself is legacy nor abandoned. I personally am just a very simple Git user and use Github through the git command and for some tasks through the website of Github. It's fine for me and I don't care if someone calls it legacy or abandoned. It's not.

When she says it's starting to feel like legacy software, I think she means parts of it seem to be falling into disrepair. Some things that once worked consistently and easily, like using the browser's built-in search, no longer do.

Legacy means outdated. Not [necessarily] unusable or unstable or insecure or needs to be updated. But feels old or outdated. Conforming to older standards or workflows.

Wikipedia matches my understanding:

In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system", yet still in use.