Currently, if you look on the wikipedia article for a given tube line it will list its ridership for the year 2011/12, but nothing later. It seems this was the last time that TFL released this data publically.
Here in the UK however we have a fantasitc law - the Freedom of Information Act 2000 - which allows us to request data from any public instituion. Thus, for completeness I submitted a request to TFL to get the data. They replied with a pretty interesting spreasheet detailing the patterns of line usage in 2018.
Soon I will update the wikipedia pages but for now you can get the data here:
I am currently a little confused by some of the data, as you can see in my final reply to the FOI. If anyone knows how to read this data better than I, let me know.