I think the answer to your question is already in one of the SO posts you linked to. Unfortunately, the deprecation of singleLines is not a black-or-white matter.
It is deprecated, but it is not going anywhere anytime soon.
It was deprecated because its performance is poor, relative to its successor, maxLines. It uses SingleLineTransformationMethod to replace newlines and carriage returns in the String you place in the TextView, unlike maxLines, which “just” adjusts the height of the TextView based on the number of lines and does no String replacing.
This method of replacing characters also meant that singleLine could break in unexpected ways (e.g. if you use custom fonts). It was these performance and reliability issues that led to its deprecation.
However, it is not going anywhere because, as the SO post you linked to states, it is still in use by many old Android applications, and it is still useful sometimes (e.g. when you want to show the entire text on one line and ignore carriage-returns and newlines).
Do note that deprecation does not necessarily mean that an API is going away. It just means that its use is discouraged, but may be permitted.