Why? LSTM as such isn't patented, so the concept is free to use by anyone without any restrictions or royalties, no matter who invented it.
Also, the concept of LSTM wouldn't be patentable directly even in the current liberal software patent interpretation, you might patent particular applications of LSTM (e.g. this patent) but not any and all arbitrary applications of LSTM.
Also, the concept of LSTM wouldn't be patentable directly even in the current liberal software patent interpretation, you might patent particular applications of LSTM (e.g. this patent) but not any and all arbitrary applications of LSTM.