What is the meaning of the literature references?

They list the sources upon which the description is based. It certainly does not pretend to give an exhaustive list of the literature about the species in question.

Why is “my” plant not mentioned?

Almost always because the plant is discussed in the site is under a more modern name. For practically each scientific plant name there exist a number of synonyms. It is not possible to incorporate them into the site. As far as possible the botanical nomenclature in the site adheres to the Euro+Med PlantBase, and with a check there most of the times the current name of the plant is easily found. If this approach fails (ornamental plants, hybrids) often The Plant List brings a solution.
But as a matter of fact, not any plant that grows in Europe is discussed. Greenhouse plants, indoor ornamentals, uncommon ornamentals, exotics on botanical gardens, and cultivars are not dealt with.

Where can I find the old website

The old website is available on https://bladmineerders.nl/html-site/index.htm.
Please note that this website has not been maintained since 2017.