Some Vector Libraries may not work in 5.0.0.
Version 4.4.5 (and previous versions) is very tolerant and permissive when it comes to accepting a vector library .svg file.
Version 5.0.0 insists that the .svg content has clean and pressed clothes and has clean and polished shoes.
Here is a fine example:
Free Vector Library based on Feather Icons
That vector library works ok in 4.4.5 despite the fact that its first line is actually malformed and it has no defined title. (It is possible to fix this but it’s not a pleasant task.)
In 5.0.0 that library is unacceptable and is not shown, for both those reasons.
There may be many vector libraries that are currently in use, either by downloading from somewhere or that were made by users who will be very disappointed when 5.0.0 doesn’t show their vector libraries.
One way to regard this would be to say, “If it’s not well formed and has no title, it’s not coming in and it’s your own fault.”
Another way would be to at least allow a vector library with no defined title to be accepted and displayed by its filename, as it is in 4.4.5.
In Inkscape, it’s quite easy to forget to give a vector library a defined title when you make it so this may be a common problem in the future.