In the github issue: Longtable change breaks 'X' column specification (https://github.com/latex3/latex2e/issues/111) the problem of the incompatibility of a necessary bug fix in latex3/latex2e is given.
In the stackoverflow issues /answers:
some solutions are given. The "problem" with these solutions is that when using them with current (old) LaTeX versions they break those versions. This is of course a problem for developers of (open source) packages that output LaTeX code.
The question is: is there a possibility (test / if statement) so that the solution code can be included and it will work in the old and new version of LaTeX or is there any possibility that the cloning and fixing of the tabu package (as suggested by Frank Mittelbach in the mentioned github issue) will take place before the next official release (TexLive 2019)?
Note: I posted a comment with some of the mentioned issues, but didn't get an answer yet so I decided to make a question out of it (also for clarity).